It lets you leverage it while physically outside of the network (eg at a hotel)
But NextDNS isn’t on your network anyway. You can access it from anywhere.
You don't strictly need it, it just makes it a tiny bit more convenient since you can set it up to override DNS on any connected device, and Tailscale sets up a private VPN mesh between your devices I've come to get take for granted - a tangential feature that goes well with centrally managed DNS.
And also benefit from Tailscale drop feature
Is there a tutorial you recommend?
There's a lot more to Tailscale but for a basic setup you just install the client on all your devices, and set DNS to the NextDNS endpoint. Any device on your network will automatically pick it up.