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.
The reason, fundamentally, is that the average chess game lasts about 40 moves. therefore, in most cases, the game is over in 4 hours at most. If it goes beyond 40 moves, then it is over in 5 hours at most. It has nothing to do with adjournments, everything to do with game scheduling.
There hasn't been adjournment chess at the professional level in decades.
Time controls have historically been specified as some duration, plus some whole number of hours [often one] to be added after certain numbers of moves, because chess clocks were traditionally constructed by taking ordinary clock movements and adding mechanisms to selectively stop them, along with a flag that would fall when the minute hand reaches "12". If a time control is 2h40 plus one hour to be added after move 40, and a second hour after move 80, but no extra time beyond that, then both clocks would be set for 3:20 at the start of the game, and a player would run out of time if either:
Note that at no point in the game would the player's clock be adjusted to add the extra time. Instead, the players would simply observe what clock reading would represent the end of the game given the number of moves played thus far.
If clock makers had wanted to support adding 30 minutes at a time instead of 60, that would not have been difficult. All that would be needed would be to replace the minute hand with a double-ended version which would simultaneously point at e.g. 12:00 and 6:00 but still rotated once per hour. If enough moves have been played to extend a game by 30 minutes, the game would end when a player's flag falls and the hour hand is between the 6 and the 7. For a tournament to impose such a time control, however, it would be necessary to have all players' clocks support it.
Time controls were beginning to be used in the mid 1800's, with clocks becoming a regular appearance starting in 1861.
The very first tournament where the double-sided chess clock was used was London 1883, which set a minimum limit of 15 moves per hour, failure to do so resulting in forfeit of the game by the player who has exceeded the limit. The games started at noon and the playing session would continue to 5pm, after which–should the game not yet be concluded–there was a two hour interval for supper followed by the adjournment (until 11pm, if necessary).
Source: Kevin Spraggett
Additionally, he mentions how there was no standard time control until later, when 40 in 2.5 hours, followed by 16 or 20 in an hour became commonplace.
While preventing players from thinking too long over individual moves was one of the reasons for the introduction of time controls, adjournments were also a consideration.
As @Saibot mentioned in a comment, deciding on a new time control isn't easy. Also mentioned in Spraggett's blog, players are reluctant to change the current time controls.
