My question is not "how does federation work?" It is "in what situation is federation useful for video conferencing in a way that a single open source server is not?"
Depending on the network topology and number of callers, you could get some benefit from a multiple server model. Large meetings across offices could make use of a server at each office to streamline bandwidth. Not a good fit for today's reality of everyone from home or your scenario of 10 callers. Could still be useful for EU vs US callers to connect locally and only one stream across the atlantic. Multiparty e2e calling is already challenging, and adding multi-hop routing might make it more so, though.