1) It obviously varies w/ each dev's situation, but I think your sense of the value prop is a fair default one w/o context. In my case, there were four reasons: 1) having a partner to help with (to me) "the unknown unknowns" (i'm only a dev and a novice designer, literally anything else would be my first time doing it, so i figured having an experienced partner there would be wise) 2) the advance was nice just from a "bird in the hand" mindset 3) they helped me connect with a really good artist (critical to the project's success imho) 4) having a big name behind you can't hurt, and bigger publishers do have more relationships with the ecosystem (streamers, platforms, etc) to help your game succeed.

2) EA is appropriate for certain kinds of games. But I knew exactly what Ballionaire was going to be, and felt it could be achieved in a year. I knew that this mechanical space was going to be rapidly saturated, once all the games inspired by LBAL (including Balatro!) started appearing. So I was determined to get the game out before that happened, which set a certain scope and pace, obviating EA.

3) The trailers were a really good promotional tool, very effective. But most of the paid promotion was sponsored streams, not traditional "marketing" (ads). I think the game has a natural tendency for organic spread, due to its fun/simple premise, watchability/streamability, low price point, and so on. It's just an easy game to see, and say "ooh, I wanna try!" because it's instantly understood how to play, and IMHO is very inviting aesthetically.

My takeaway: Make sure your hook is glowingly radioactively good. Don't overbalance. Leave in some jank. Scope down and finish quickly. And avoid tropes. Stand out. (Of course this only works for a certain kind of game!)