The concept is well tested and solid; have your main story supplemented with additional works, such as comics and novels, that expand on the core work. This sort of franchising has become extremely popular to the point of being a standard practice and very few standalone projects get made any more. Even the Marvel Cinematic Universe has seen its stories expanded in aforementioned media. This always leads to the question of canon, where the main piece always trumps over whatever the side material has stated. Hence why canon barely matters, when anyone in charge can say what really happened, sometimes wiping few comics away, sometimes erasing whole decades of supplementary material.
Nevertheless, they’re secondary at best. Licensed works to make some money out of the IP while the main thing is wiring its next stuff up. The stories and characters told in these works don’t really matter, and never have. Only decades later, when fans who grew up with these materials, may make references to them in proper works, giving them some legitimacy in the eyes of fellow fans. That’s all fine and dandy, no harm done by having someone in the background mentioning Life Day and reminding the people in the know how bad Star Wars Holiday Special is. It’s butt of the joke, it’s done to death, we get it.
Star Wars and Star Trek are great examples of this as both have extremely extensive supplementary material to go with the main works. The general rule has always been that what’s on the screen overrides whatever’s in other works. While they’re advertised as further adventures of our heroes, and for the time being they probably are, they’ll always be overridden when the IP owner comes up with something new, something that can be capitalised on. Prequels and midquels are sort of comfortable ground to many, as they’re mostly based on sayings and history told in the main works, so it’s easy to take the premise and go town with it. It doesn’t exactly require creating something completely new from the ground up. Hence why you often see sequels lifting material from the old stuff or reusing characters and settings. Jean-Luc Picard and all the re-used assets from the cutting room floor in Disney Star Wars movies are examples of this.
All this a somewhat inconvenient arrangement, but it makes two things possible; it doesn’t demand the audience to rummage through hundreds of pages to understand a new TV-show or a movie, but also allows them to engage with the IP and characters further. There’s this silent agreement with all the parties that it is probable that all the side content will be ignored when a new movie or show rolls in. Which happens all the time with pretty much every single large franchise out there.
There are of course times when this fails. The Rise of Skywalker had a collaborative event in Fortnite, where Emperor Palpatine’s speech was introduced. The returning villain and one of the major points of the plot, which was used in the beginning crawl, The Dead speak, was introduced and used in the aforementioned event. This effectively cut a section out from the movie, the message Palpatine send to the galaxy to announce his return, something the characters all react to and is the impetus behind the movie’s events. If you weren’t playing Fortnite at that time, you effectively missed something the core work, the film, should’ve had.
Too often Star Trek and Star Wars novelisation have been used to correct mistakes and loopholes in the main body of works. Loads of Trek novels based on The Original Series episodes were used to effectively fix continuity and conceptual errors within the episodes themselves. Similarly, The Rise of Skywalker‘s novelisation reveals that the Palpatine in the movie was a clone. Whether or not this is canon is of course for the fans to debate, as none of the corrections and fixes are rarely talked in the main body works. It’s not uncommon to see books and comics being published that fill in holes with some plot putty, sometimes even explaining whole backstories and events that were completely lacking from the main works. We can understand that a movie can’t set up decades worth of background story in a short time, and sometimes it doesn’t need to. The original crawl at the beginning of Star Wars Episode IV is work of sheer genius, setting up the premise. Further into the movie, short discussions about the Clone Wars as a background material elaborated on some bits, but those there to colour the world further. With Disney movies we have the gap between Episode VI and VII, which is just a void. Even after the last movie we barely know what happened, where did the First Order truly come from and why did the Emperor allow the Empire to fall just to wait thirty years building Star Destroyers under ground with gimped navigation systems. Maybe it’s Abrams’ mystery box killing the work again, maybe it’s just outright bad writing. These explanations of course are found in the supplementary material, meaning the work can’t stand on its two feet.
You could of course argue that this weaves the main work and the supplementary works together better, that it allows exploration of these events and concepts in a grander scale compared to what movies and television could. This is completely true and has been supported by multiple franchises for some decades now, mixing and matching each other punch to punch. The problem is of course the future. Be it removal of old canon or a new “real” work taking place of that timeframe and overriding the current works, supplementary material never really can stand the test of time. Not unless the creators are adamant on keeping one continuity and will always take notice what happens across the whole franchise. That task is nearly impossible, though if you were to hire bunch of people just to follow what the hell’s going on in your setting across all media, it would become manageable. Imagine if your day job was to read every Star Wars book and comic just to tell the future writers of whatever series or movie they’re making what stories and settings have already been used, and what are their historical consequences. Somebody’s dream job right there.
While you could boil this down to Canon doesn’t matter because it always changes, but that’d be missing the point here just slightly. We’ve seen the main work been put on the chopping block and some of its important elements have been cut off, only to appear elsewhere. This weakens the main work, but it also makes the story’s canon that much weaker. If you’d need one more example of this, the 2009 Star Trek reboot movie doesn’t ever tell what Nero was doing in the past after he came through the wormhole. In the movie, he’s just sitting there doing nothing and waiting for Spock to pop in. However in the comic he was captured and enslaved by the Klingons, making his escape and reclamation of the Narada that much more important. After seeing his home world destroyed, seeing Vulcans’ inaction as betrayal despite putting everything he had in their hands, and then forced to the past and for years being unable to do anything to prevent that from happening, Star Trek could’ve had its best and most understandable villain. All that was from the movie, making him just a jackass with a vengeance. It’s only a matter of time before someone writers a new book or a comic that explores this further, erasing already established events in the comic, which already is questionably canon. The comic version’s story is that much stronger compared to the movie, but it’s the not the story. It’s just an alternate take, which some people supplement the movie itself with.
Here’s a way you could make cross-media function for their own benefits without taking away any from the main work. The Mad Max game from 2015 was supposed to be tightly connected to Mad Max Fury Road, but ultimately wasn’t. The two would have supplemented each other, but only in a manner that there would not have been anything missing from either work. For example, the Pursuit Special is missing its spoiler in the movie. Not a huge detail, something most people probably missed altogether. However, in the game it would’ve been a collectable item with some story tied to it, adding to the overall story of Mad Max. We don’t know what these details were going to be, as the game was completely revamped and reused two decades worth of abandoned concepts alongside concepts for possible future movies. If you’re a fan of Mad Max, the game should look and feel extremely disjointed and somewhat schizophrenic because of this.
This is really convoluted and lengthy way to say how works, even sequels, need to be standalone enough to be consumed as-is without any surrounding media taken into account.