Personnally, I need them to run a game smoothly.
I need the screen to ensure a smooth run of the game, so as to avoid flipping through the books.
I need the Starships book because my campaign is heavy in ships stuff and technology.
I need the Starfleet Ops book because my players are Starfleet officers and run their ship.
And I don't have days and days to make all the stuff myself.
I'll want - and buy - all the rest as well, and I'm sure other GMs need these other books.