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.
This is because I got used to butter on all of my sandwiches, because my mom made them that way. The reasoning was the butter prevented the other wet stuff from soaking through the bread, and that works.