There are two reasons a player would join the waitlist:
The game is full (you've set max players and/or players) and a player wants to indicate that they'll play if someone drops out.
The player is a spare and they haven't yet been invited to a game. They can join the waitlist at any time to let the managers know that if they need someone, this person is available.
The manager can accept or reject people from the waitlist at any time. You can use the auto waitlist feature to automate the acceptance process.
Learn about the automatic waitlist here: https://help.benchapp.com/en/articles/5155091-how-does-the-auto-waitlist-work