I’m not sure what the popular use case is for multiple connections to one multiplexer. But, two (niche seeming) ones could be: if you have a desktop, you want to be able to SSH to it and use it locally at the same time. Or, if you have two people ssh-ing to one system, and letting them share a terminal might be nice (although in that case it would really be nice to give them independent cursors, which starts to become an involved project).
I usually have a drop down terminal that I use the most, then a full screen window on some other space. I can switch between them quickly and connect to any of my sessions from either.
Or one person ssh'ing to the same remote from two or more devices. If I don't feel like sitting in the office (desktop) and grab the laptop and go to the sitting room or back deck I can continue my session without issue, and then transition back later. I don't want to have to disconnect/detach a session when I do this, I want it to be seamless so both connections (actually three typically, an iPad as well) are running continuously.
we do use multiconnections when doing interviews - candidate solving test case on VM, interviewers are observing. Idea was not to confuse candidates by requiring screensharing during the interview, just see the particular ssh session.
