Since it took a while for me to figure out, I will leave this here for posterity
Use case: You want to be able to mount a networked (SMB/CIFS) filesystem on Linux on startup but that endpoint needs to wait for Tailscale to be up (for instance, in a Synology NAS).
Make sure you specify the options noauto,x-systemd.automount,_netdev in /etc/fstab.