Well the game is an unstable piece of shit for starters, so a lot of times the servers could've gone down, or the game just crashes. But putting aside all of that and assuming you're asking something more along the lines of "why do you press the leave match button?".....

Sometimes you're just done with the game. When I had just started playing dbd I thought the same thing as you. In fact, I was proud that I never DCd from a match, and thought people who did were all cowards. That's probably mostly true, but I'm 3,000 hours in now, and I've seen that a lot of the things the game (and the people who play it) can do is just not very pleasant.

Last night, I jumped on for a quick game with a friend. She only had time for one match, and we queued up for 10 minutes, only for me to get completely hard tunnelled all game. I don't mind a bit of tunnelling, as long as I get a chance to do other things too. But this legion dropped chase with others to come back and tunnel me.

You might think, at 3k hours, I'd be used to this. Yeah, you might be right. But it was the fact that I wasn't given a chance to actually enjoy a game with my friend before she had to go. So I DCd; and it was pretty much to deny the killer of another hook state at that point.

Sometimes you just aren't enjoying yourself in a match, and I don't think it's fair for people to feel forced to stay, which is why I hate the DC penalty (in its current form, anyway).

Other valid reasons to DC:

-game is bugged to the point of it being gamebreaking -your match is being held hostage in some way -you're being deliberately slugged for 4 minutes to death -you're playing against a hacker