I recently found Reefbane, a unique "fishing rod". I cannot understand its usefulness, and I am under the impression that it is an "inside joke". Is this right? Or are there some legitimate uses for this item?
Fishing is an ongoing joke in the game. You can't really do anything with fishing rods or fishing items except collecting and/or selling them.
Fishing items are extremely rare and popular among collectors. Some rods sell for multiple exalted orbs so they're not completely useless if you find a buyer.
Always check poe.trade for the current price of the rod in your league.