Currently, multiplayer rooms can only be locked if they're "full" (at the max peer count). But there are many instances where the host would want to lock a room with fewer players. For example, starting a game with three players, despite the max peer count being four or more. Or if a player disconnects while a game is in progress, you don't want them rejoining until the game is over.
Giving hosts control over when a room is locked/unlocked will help many multiplayer ideas come to life that can't right now (not without some complex workarounds, anyway, which feels very non-construct). There have been some forum discussions in the past that involve heavy work to keep players from joining rooms.
The lock/unlock feature currently exists, so we know that it's important. This idea is to remove its limit by being tied to peer count, and let the lock/unlock feature be used at the user's discretion.