depends if you ask about lifer one-night stander or occasional.

For the occasional you've had good responses already.

As for the lifers, there are people that cannot come too close, cannot share some real intimacy with their partners.
The only way out of solitude is a one night stand.
It's not much, but they something is better than nothing and they can get the idea that they are ok, feeling great and enjoying life at its most.
Except they cannot really afford a psychological bond