I'm not familiar with Magic Wormhole specifically, but I think this may be to solve STUN: https://en.wikipedia.org/wiki/STUN https://en.wikipedia.org/wiki/STUN.

I'm not familiar with Magic Wormhole specifically, but I think this may be to solve STUN: https://en.wikipedia.org/wiki/STUN https://en.wikipedia.org/wiki/STUN. Specifically if the clients are behind NATs, having a third party server coordinate can help.