It’s like falling back to hub and spoke, except that the traffic is end to end encrypted, and the middle node is used only when direct connection is not possible, and for some clients. It’s also similar to running your own derp server (which works also in TCP), but without the hassle of doing so, and perhaps without having to open ports to the internet (needed in derp) so long as the relay is reachable by peers.
The derp servers have low throughput. Another option could be a pay-as-you-go derp service.
They might also be on their way to remove the need for reverse proxies, with the recent announcement on Tailscale services.
BTW, why could it be paid for more than two relays? You are using just your own devices and bandwidth :)
It actually lower the bandwidth bill for Tailscale by reducing the usage of their own relays. Ideally, by default the software will find whatever nodes could help with direct connection. It’s just routing within your own network.