1. Obvious reason to have casual sex: Feels good, instant gratification, etc.
2. Non-obvious reason to: The boost in self-confidence that comes from knowing that another person was attracted to you physically. Casual sex is about physicality. People need validation that they are beautiful. People who think they are beautiful are more self-confident in life. Self-confidence is good.
3. Obvious reason not to have casual sex: STDs.
4. Non-obvious reason not to: You usually feel lonelier afterwards.
I believe #2 is the most original of the four.