Banana and sugar is the one banana sandwich whose defining feature is grit. Where banana with honey or custard adds smoothness and moisture, white sugar sprinkled over the fruit stays partly granular, and that crunch against soft banana on soft bread is the entire point of the build. It is a textural sandwich disguised as a sweet one. The pleasure is not really the extra sweetness, which a ripe banana barely needs; it is the contrast of crystals that have not yet dissolved against a filling that has no texture of its own.
The craft is timing the sugar against its own dissolving. Sprinkled on dry it stays as distinct grains and gives the crunch the sandwich is made for; left too long it draws moisture out of the banana and turns to a syrup, taking the texture with it and adding to the bleed the fruit already brings as it browns. This is why the sandwich is built and eaten quickly rather than packed: the window where it works is short and on both sides of it lies a damper, duller sandwich. Butter spread to the edges does the structural job, waterproofing the crumb against the banana's weep and the syrup the sugar eventually becomes, and salted butter supplies the savoury counter that keeps plain sugar from reading as flat. The bread is soft and plain so it yields to a soft filling rather than fighting it.
The variations are small and mostly swap which sugar does the work. Demerara or another coarse brown sugar holds its grit longer and brings a faint molasses note, trading some sweetness for more durable crunch. Caster sugar dissolves fastest and is the least textural choice. A scrape of jam under the banana pushes it toward the plain jam-and-banana build. Each deserves its own article rather than being crowded in here.