See Attendees currently in a meeting before joining or resuming
There are several use cases under which members of an organization might want to view a list of attendees currently in the meeting before actually joining (or resuming) a meeting.
Below are 2 examples.
Anyone who finds themselves overbooked and invited to multiple simultaneous meetings might wish to see if a particular attendee is in a meeting before joining as a way to prioritize which meeting they will join. In other words, the person one most needs to talk to or hear from either is or is not in the meeting.
Teams create 'open' meetings (meeting rooms, essentially) in which team members could come and go at will throughout the day.
In this second scenario, not only might one want to view who is in the 'room' before entering, one might also have placed a meeting on Hold while attending another. Before resuming the on Hold meeting, it can also be helpful to know if certain team members are currently in the meeting.