Sometimes you want two computers to be able to talk to one another, even if they're behind a NAT or there's otherwise infrastructure between them that makes this difficult.

Zerotier makes that easier by

* Checking to see if the other device is directly reachable, if you're both already on the same LAN.

* Employing NAT hole punching to try to establish a path anyway

* Finally proxying the connection over a server reachable by both

In my experience it's the easiest way to make sure all my computers are reachable, even if one of those computers is a phone and another one is behind a NAT.