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.
It tends to break when you force power off the machine in my experience, where ext4 is super resilient to that kind of stuff.
Thats my experience at least.