It solves the key distribution problem. If you have multiple Wireguard hosts in a mesh infrastructure, it can be tricky to change or remove a key quickly and consistently. No benefit if it’s only a single tunnel between 2 hosts.
It provides STUN/DERP services to connect hosts behind firewalls or NAT, without opening ports or redirections.
Tailscale also provides more advanced services or configuration helpers, such a file sharing (in alpha), ACLs…