If you can setup the required port forwards, let alone the ssh tunnels, then you don’t need the relay functionality. You’re already set. This is how it is today. However not everyone has the gear, know-how and inclination to do this. The relay functionality is for them.
@LinuxBaby Newbie here.
Actually, you can run Syncthing over N2N. Of course, the data might be relayed through a public N2N supernode.
I have a few computers (all behind NAT) connected together through an N2N network. Recently, I installed Syncthing to a few of them and I realize that it is using N2N interface to connect to each other. If I disable the N2N interface, Synthing stops seeing each other. Therefore, I let it be for the time being.
I agree with you that if this capability can be implemented into Syncthing, it will be really convenient. Or, maybe make Syncthing compatible with N2N supernode so that we can “piggyback” them… (joking)