I made it because my friends and I use wireguard to have a private network, and they don't feel comfortable running a ssh server on their machines. It can also be used with a vpn provider like Mullvad without setting up a new nic or requiring special privs.

One simple reason is that the serverside might not want to expose a shell to its clients, and instead just provide network connectivity; you can configure something like that with SSH, but it's a pain, and WireGuard is approximately as simple to set up as SSH, which is the primary reason it's so popular.

Presumably you'd use it on a machine that doesn't expose SSH. That said, I really like sshuttle for this use case when SSH is available.