There is a point where the paddle rack stops working. People wait too long, the same four keep winning the court, and newer players quietly stop coming back.
A session is the middle ground between open play and a tournament. Same drop-in crowd, same night, but everyone gets scheduled into real rounds with rotating partners, scores are tracked, and there are standings at the end. It takes about a minute to set up and players do not download anything.
It works for 8, 9 or 18 players, and up to 21 in Turbo mode. That is the size where open play starts to strain.