"Necessity is the mother of invention... Boredom is the father." -- SilverRey
So many inventions happen because their inventor thinks that they are vital. Others occur because they will create a tactical advantage against an enemy. Some rare and interesting inventions happen because the creator had nothing better to do.
Case in point - weird vehicles.
People don't need a bicycle that walks on feet, nor do they need a chariot drawn by crab-like legs. They don't need a powered boat with oars that operate like turtle flippers. That said, once they are built, everyone seems to want one. Well... Everyone who's a Human.