I never understood it myself, but over the years I've seen many people (mostly kids) walk up to bikes and put their hands either on the grips or the tank. On several occasions, I've found mystery prints on my tank, and I never have any reason to touch it.
+1 on the mystery prints. I think my daughter has been showing off the bike to her friends while it's parked in the garage.
Most of my friends are combat vets, one of them has a sticker on his that says "This bike owned by a Combat Veteran, f*ck with it and you too will know how it feels to be shot at"
Pretty much guarantees no one will touch your bike.
Sometimes a "Support your local Red & White" sticker will add a nice touch to deter those who would otherwise mess with your bike.