Saves water if you think about it. If you wait for the water to warm up before washing your hands, you can just piss in the sink while the water runs and warms up. Also you're not flushing a whole gallon+ or water per piss.

I always piss after cleaning the dick after masturbating. Pissing after fondling yourself is a good idea to get the pipes cleaned out thoroughly, and cleaning the whole thing is just good hygeine.