One of the most potent avenues to get invested in a game is through our creations. It's easy to care about things we've made - they often express something personal about ourselves, whether it's something deep or just an inclination.
When people praise them it's satisfying, and when they take on a life of their own it's especially cool. There's nothing so memorable as seeing one of your creations celebrated (or booed!)
For players, however, once character creation is done, the canvas they have for this sort of creating is very small - their words, their reactions to the situation, their plans, their actions, and that's usually pretty much all they have.
Nevertheless, the group (and especially the GM, who has the largest canvas) can highlight player creations, giving them a broader effect, build on them, and turn them around into new forms.
A GM that makes players' choices consequential to the campaign world is sending a subtle signal that they care about the players' contributions: they're valuable, and have shaped the world.
Railroading and Illusionism
When a GM railroads, they are neutralizing a player creation in order to inject one of their own.
This can be done clumsily and overtly, which is how it feels when the GM does it at the level of individual player choices. The party tries to go west, but there's bad weather (in-game railroading). The players suggest going back to town, but the GM lobbies them out of character. "You don't wanna do that, do you?"
This is sending the social signal, "I don't care about your contributions."
Illusionism is sometimes held up as friendly alternative to railroading - it's railroading in secret. The players' choices appear to be relevant, but they're not. They go west instead of east? The GM inverts the map so they find the ogres planned before the session started. They go back to town? Surprise, the ogres are in town.
This avoid the unpleasant taste and resentment of overt nullification of player choices, but at the campaign level is the same.
They might not be able to put their finger on it, but none of the major events of the campaign will have come about because of their planning. For them, the most memorable stuff will be the inconsequential "fucking around in camp".
In a sense, the GM and players are creating two separate things - the characters and their antics, and "the plot" - and showing them to one another, but neither was meaningfully shaped by the other.
Good For The Goose
My favorite objection to illusionism, though, is to reverse the scenario: player illusionism!
Before the game, the players get together and plan out how they're going to render a pre-planned dynamic, like an intraparty conflict. Whatever the GM throws at them, they'll carefully neutralize as best they can, ideally without the GM noticing. They'll converse with NPCs, but they'll be trying to ensure the GM has as little impact on their plans as possible - won't it be awesome?