It's always been my assumption that we wash our hands when we go to the bathroom for two reasons:
1. Wiping your butt can lead to feces getting in your hand. A lot of diseases can come from feces.
2. It's a good frequency to wash off whatever else you've been touching that day.
Assuming you go to the bathroom 3-7 times a day, that's that many opportunities to wash off the dirt you were digging in, trash you picked up off the street or someone else's hand you shook.
Assuming you go to the bathroom 3-7 times a day, that's that many opportunities to wash off the dirt you were digging in, trash you picked up off the street or someone else's hand you shook.