re: What's the purpose of this boat?

Tower boat. Not a poling platform but used for sight fishing. They usually have dual controls up in the tower and down below. Saw my first one at Boca Grande ten years ago. Lot's of sight fishing in that area with gin clear water for reds, snook, or whatever. Now I'm seeing more and more towers on Choctawhatchee Bay. Great for spotting cobia along the beach (Destin) on their spring migration. Lot's of guides I've fished with in Florida actually SEE the fish. Crystal clear water has its bennies. But then again the fish can see you.

That's definitely a fishing boat. Notice the rod holders and no drink holders.