I'm not sure of my source, but I believe the majority of chess games last around 40 moves, or go into a longer end game, which could take the game from 60 - 80+ moves (depending on the complexity of the end game).
I assume the second time control is typically at move 40 so that the initial time control doesn't punish someone simply because their game went into a complex endgame, but rather, gives more time for the aformentioned end game.
I was just wondering why in a 2 hour time control, with an extra hour at move 40 to finish the game, why wouldn't you simply start with 3 hours (each)? I know it is very common to have time controls similar to this one.. but why do tournament directors do this?