Don't get me wrong, if you have a good jam or preserves it's totally fine on a sandwich, and I wouldn't turn my nose up at it. However, peanut butter doesn't really need the help, and the extra sweetness from jelly is totally unnecessary. Toasted bread and peanut butter is all you need for a quality simple sandwich.
Butter on sandwiches isn't something I grew up with, but I can appreciate the logic behind its use. I've tried it a few times and am generally neutral on it. It is in essence—on a sandwich infrastructure level—mayonnaise with a different flavor profile.
I'd expect peanut butter to be a better moisture barrier than either butter or mayo however.