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.

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.

And also benefit from Tailscale drop feature

So people with access to the TailScale control plane can easily add and remove devices from your network.

https://youtu.be/bJHPfpOnDzg