Well movie soundtracks just have to make the dialogue less boring and the action sequences more exciting.

Video game music is something that sticks out like a sore thumb (my mother hates it and she doesn't even play any games so it must be pretty damn noticeable), so it's absolutely essential that it be any good.

As it happens, I agree with the sentiment being expressed by this topic's title, though of course there are plenty of games with severely sucky soundtracks. Saying that there are plenty of movies too: GoldenEye (the film, not the game; that was legendary) had some particularly disastrous numbers that just sounded way too cheesy. One critic actually mused that the music was better suited to an elevator than a movie, but that's what you get when you let Bono compose your theme song, then the pompous asshole says "Okay, now you're not allowed any remixes of it in your film".