Bananas reproduce in a similar way as green onions. They eventually grow another rizom that is a clone of the original plant. You can force bananas to produce seeds but the seeds produced by most slightly older methods lead to seeds that germanate maybe one out of 10000 seeds.
As for why there are seeded bananas... I would think it would be obvious. After weed the next crop to be cultivated was bananas and it took a lot of work (by whoever did that work) to change them from something alot like a vanilla pod to a banana.
Most, if not all, of the "wild" bananas are early seeded varieties that were eventually abandonded in favor of better genetics but the seeds and plants thrived without us. Sound a little like hemp? It is, but older.