The longer version
A ladder is the simplest competitive format a club can run. Players are listed in order, a player may challenge anyone up to a set number of places above them, and if the challenger wins the two swap. There is no fixed schedule, so a ladder runs for months without anyone organising rounds.
Clubs like ladders because they create a reason to book a court on a quiet weeknight. Two players arrange their own match, pay for the court, and report the score. The ladder does the matchmaking that would otherwise need a coach or a group chat.
The common failure is inactivity at the top. If the top 5 never accept challenges, the ladder freezes. Most clubs add a rule that an unanswered challenge after 14 days counts as a forfeit, and that a player who has not played for 30 days drops a set number of places.