I find it really useful as a (nearly) zero-config way of accessing devices behind a NAT box.
Even my work internet connection is behind a carrier-grade-NAT box so I can't even port forward. I use it to access our Jenkins server which is physically located inside the building.
It's particularly useful for 2 reasons:
- each machine gets a very stable IP (unlike DHCP-assigned addresses) which simplifies hosts files / dnsmasq etc
- it's better than port forwarding as every device can work on port 80
So my use case would be something like "simple LAN setup with no more port forwarding"
https://www.zerotier.com/