Some people might have more than one subnet. Or road-warrior VPN and VPN-ing in. It is nice when your networks name resolution works, even if you are outside your network.
If you have multiple subnets, you almost certainly have an advanced enough router that it supports mDNS forwarding. There's no reason why it can't work across a VPN either.
mDNS forwarding doesn't work on Tailscale :(
mDNS has some downsides that can be important for some. For instance, the fact that you can more easily set the same host name on multiple machines, or the fact that it uses a lot of bandwidth.