Funny packets make things behave funny sometimes. Sometimes you just need to see how something behaves when you send it illegal packets that the real software would never send.
It also makes it possible to cheat in some games by lying to the game server about interactions in game.
Essentially hackers need a way to talk to machines at every level of every protocol and Scapy is a pretty standard way of achieving that.
I wish it would have 2 ports, top and bottom, so I can be more creative with my accessories.