I think you answered the question. Sugar. It's easier than managing your own Wireguard connections. Adding a device just means logging into the Tailscale client, no need to distribute information to or from other devices. Get a new phone while traveling because yours was stolen? You can set up Tailscale and be back on your private network in a couple minutes.

Why did people use Dropbox instead of setting up their own FTP servers? Because it was easier.