Annoying in general but the case that was the worst for me were the road checkpoints in far cry 2. Would clear them out, turn around and the enemies would respawn already

It's to give the illusion that there are tens of thousands of enemies.

I hate it when you don't know they infinitely respawn and you try to clear them out so you can look for items in peace.

It really depends on the game. Sometimes it is done really well. Other times it's infuriating. Some games basically need infinite enemies so you can grind EXP and loot. Other games do limited spawns really well.

It ruined Star Wars Bounty Hunter for me lol.

it depends on what type of game it is and how well the repopulation is executed. it's pretty much impossible to repopulate areas with unique enemies and when they refill the map with the same enemy or scaled enemy or use randomly generated enemies the experience feels cheap and unimportant. i don't mind instanced games like path of exile where each time you go into a map there are new enemies to fight but you know and expect that to happen beforehand and it's easier to avoid boring encounters when you can be selective about repopulation.

Depends on the genre.

I think I gave up on system shock 2 when I got hit from behind by a zombie. Can't be bothered looking 180 all the time when I'm trying to manage my inventory.

I think its also in Mechwarrior 5? Not happy when you can't clear an area and just enjoy the scenery and do mission objectives in your own time.

Exp farming opportunity, it can be annoying though

It's an annoying, stupid mechanic.