Some do. My grandmother always did. Some use mayo or whatever the stuff that isn't mayo but is sort of like it is called. (I am blanking on the not-mayo stuff at the moment.) Others use mustard.

Assuming of course this is savory sandwiches we're talking about. I don't think I've ever seen a fellow American put anything but peanut butter and jelly on a peanut butter and jelly sandwich.

It also occurs to me that I have no idea what a sandwich with butter would even taste like. (I have always used mustard.) ???