Peanut butter and banana is the peanut butter sandwich with the cling problem answered by soft fruit, and the banana is the whole reason it works. Peanut butter alone is dry, adhesive, and relentlessly savoury; sliced banana laid over it brings moisture, a mild sweetness, and a yielding texture that the peanut butter has none of, so the two stop being a single heavy note and become a contrast. The defining fact of this build is that the banana is doing structural work, not garnishing: it is the wet, soft, sweet counterweight that lets the peanut butter read as rich rather than as paste, and without it the sandwich collapses back into the bare version's clag.
The craft is moisture and timing. The peanut butter is spread to the edges first because it doubles as a seal: a continuous fat layer keeps the banana's moisture off the crumb so the bread does not go to a damp slick before it is eaten. The banana is sliced thin and laid in an even single layer rather than a thick wet log, because a heavy bank of fruit slides under pressure and squeezes out the sides the moment the sandwich is pressed flat. This is also the one build in the set with a clock on it: cut banana oxidises and turns soft and grey-brown within the hour, so a peanut butter and banana sandwich is at its best made and eaten close together, not packed in the morning for a lunch hours away. Soft white bread is the convention because the filling supplies all the texture the bite needs and a chewy crust would only fight it.
The variations stay inside the soft, sweet frame. A drizzle of honey pushes it sweeter and stickier; a fried version warms the banana and melts the peanut butter into it; the wider sweet-sandwich family of jam, honey, and chocolate spread keeps the same buttered-soft logic. Each deserves its own article rather than being crowded in here.