Fuel selector off is for two reasons...
One, if the fuel system downstream has a leak, your fuel will leak out on the floor. Less of an issue on tailwheel models, more for a tricycle gear with full tanks i'd guess.
Two, if some looky-loo spins your prop when it's parked, and the mags happen to be left on by accident, if the fuel is "off" it can only run for so long before it stops. If it's on, it could run until the tank is empty or the plane hits something or someone.
Lots of "ifs" have to line up for Two to happen, but turning fuel off is easy and doesn't hurt anything.