It's just a good time to wash your hands, period. You touch so many things on a regular basis, why not just wash them post pee? It's just good hygeine.
While I usually wash my hands after a restroom visit, I think its mostly a feelgood thing to do rather than useful in terms of hygeine. Think about what you do when you wash your hands: 1. dirty hands: open the faucet. 2. rinse/soap/rinse. Hands now clean. 3. close faucet...WAIT A MINUTE- that's the same faucet you just touched with dirty hands. So you just "undid" your handwashing. 4. Grab a towel (many restrooms have manually activiated towel dispenser, so another dirty contact point. So unless the towels have hand sanitizer on them (nearly all do not), you've really done nothing. 5. open restroom door and waslk out. Well there goes another dirty contact.
Unless the restroom is completely touch-free, you're really not cleaning your hands at all, just transferring the germs from one contact point to another. And personally speaking, I find those touch free sinks annoying. You stick your hands under them, nothing happens, so you start fiddling around under the faucet and even start touching stuff to see what activates the water- OH there it is. FIVE WHOLE MOLECULES OF WATER get ejaculated out of the faucet then it shuts off again. WTF. Hold your hands there and wait some more and see if the next squirt (when it eventually comes) is actually enough to get your hands wet. Ok, how many more shots of rediculously little water will it take to rinse the soap? Three is pretty much the minimum, SSST... wait... SSSHT... wait... ah fuck it, I'm going to dry off and get out of here already, I got better things to do, plane to catch, whatever... I understand the importance of conservation and all, but be reasonable.
Plus you're handling your dick, dude.
Well at least I know where my dick has been, which is always cleaner than where my hands have been.