The idea here is that rooms are abstracted from servers and sort-of exist ephemerally. This has the advantage/disadvantage of making it hard for the underlying infrastructure to exert control over the hosted communities, and seems to have become a distinguishing feature of federation.
My experience of Matrix as a possible replacement for Discord has led me to believe it's mostly a disadvantage since it leads to gross misalignments between the communities in top and the infrastructure providers underneath. I consider e.g. Discourse to be much healthier (although I would like to see an app for Discourse so that my Discourse communities behave more like Discord/Slack servers) and it's frustrating to me that there hasn't been a clear "Discourse for chat" emerge to replace Discord.