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.
When I grew up, bread with butter already passed as a simple sandwich. Pbj is disgusting to me, while bread, butter and jam (or honey) is good as an open faced sandwich.
The flavor of butter can be the star of a sandwich, a baguette jambon beurre in france is fantastic.
Finding the PB+J flavor combo startling, if not downright offputting is pretty common outside the U.S. it seems. Like many things food-related, preferences are strongly cultural.
PB+J is one of the few savory/sweet combinations in widespread use in American cooking (although I suppose it pops up fairly often in various breakfast combos). Others exist, but this is by far the most common. I myself don’t generally like that combination outside of PB&J, and I almost certainly enjoy it mostly because I grew up with it.