The plain peanut butter sandwich is the bare build with nothing to correct it, and that is exactly what defines it. Peanut butter goes on soft white bread and the sandwich is finished, which means the single ingredient has to carry the whole thing alone, and peanut butter is a difficult solo act. It is fat-heavy, intensely savoury, and adhesive, and on its own between two dry slices it clings to the roof of the mouth and pulls the bread to a paste. Every other sandwich in this family exists to solve that problem. This one states it: peanut butter, unaccompanied, is the cling-and-dryness test that the banana, the honey, and the jam all answer.
The craft is therefore about managing a filling that fights back. The peanut butter is spread thin and to the edges, never in a thick central seam, because a heavy load is what makes it cloy and what makes it squeeze out and stick the bread together when the sandwich is pressed. Butter or margarine under the peanut butter is the quiet fix the bare version sometimes allows itself: a thin fat layer lubricates the slice and stops the crumb drying, which is the closest the plain build comes to a counterweight. The bread is soft white on purpose, because chew would be one more dry thing to overcome and the filling already supplies all the resistance the mouth can handle. The choice of crunchy or smooth changes the texture but not the underlying problem: smooth claggs more, crunchy breaks the cling slightly with the nut pieces, and neither adds the acid or the moisture the build is actually missing.
The variations are the whole point of the set, because each one is a different answer to the same gap. Banana adds soft sweet moisture, honey adds a runny sweetener, jam adds fruit acid, and Marmite turns it savoury in the other direction. Each deserves its own article rather than being crowded in here.