Why have more than one time control per game? Why is it so common to have time controls like 2 hours for 40 moves plus 30 minutes for the rest of the game, as opposed to something simpler like 2.5 hours for the game? Is this just to "force" players to ration their time? Can't players be trusted to manage their own time? :-)

Disclaimer: I've never played under such time controls.
I think these partitioned time controls evolved naturally during the history of tournament chess for two reasons.


To limit psychological warfare. Some chess players are.. let's say difficult. I can vividly imagine the first time when a participant of an important  match refused to make his next move (or even look at the board) because he was so angered that he blundered away a piece in the 10th move but also refused to forfeit and although it was very obvious that he would just let his time run out, there was no rule yet to stop the game right there and everyone had to wait for hours. Or people started to try and abuse the lack of such rule by simply waiting for an entire hour before making the first move simply to unnerve their opponents (who were pondering over which opening they would see etc.)
To make matches more exciting for the spectators. Remember, in private everyone can set their own time control however they like. But for tournament hosts it makes sense to split it into smaller pieces to create more tension. Also, especially casual or even first time spectators might be more at ease with this because it stresses that not every game lasts this long.. you don't have to wait for full 5 hours every time. The time after move 40 looks more like the overtime from other sports rather than "regular" time.