Sure, you may know how to
wash your hands after
using the toilet, but you can be sure that about half the people who used the
restroom before you didn't bother, so that
doorknob is a
Petri dish of
filth.
If you use the paper towel as a primitive shield when you turn the knob, or push the door if it's that kind, you're less likely to put new filth on your hands after painstakingly removing the old filth.
This is probably most important if you're preparing someone else's food after leaving the restroom, like, say if you work in a restaurant.