Ordering room list by most-recently-active
People who have a long list of rooms have a hard time finding the ones they need. Most often, people would be most active in just a small subset of that whole list.
Sorting the list of rooms by "most recent activity" would make those frequently used rooms easy to find.
It also makes "unread" notifications more visible. As a new messages arrives in some "old and not recently used room", that room would get bumped to the top of the list.