You mean like a doorknob?

I think the idea is that you aren't putting your dirty fingers in your mouth, or your spit on stuff that other people might handle. The latter really doesn't matter scientifically I think, but the former probably does.

People put their fingers on all kinds of things other people have touched: doorknobs, elevator buttons, shopping basket/cart handles, etc. Adding a moist pad to that isn't going to change anything. Keeping people's hands out of their mouths, however, might.