Tailscale is just a commercial service that builds upon wireguard. It automatically generates certificates for each of your devices, ensures they're rotated and up to date, automatically configures routing and DNS between your devices and offers some additional functionality.

Tailscale has open source clients but a proprietary server to do this, but you can use the open source alternative headscale instead: https://github.com/juanfont/headscale

Convenience. you're free to run wireguard yourself, but that's a lot of faffing about with config files that some people don't want to do. and then on top of that, they have clients for mobile and eg AppleTV. that may be outside your use case, but some find it handy.

They've got fantastic documentation about this:

https://tailscale.com/compare/wireguard