That is for a Cunningham. The Cunningham applies tension to the mainsail luff drawing the belly of the sail forward. It is typically a block and tackle secured at the base of the mast and with a hook that goes into the Cunningham hole.
The main halyard also tensions the mainsail luff, however, it also tensions the leach. The Cunningham counters some of the leech tension because the pull is only vertical. The halyard tensions in 2 directions, along the luff and leech