Hi Sylvain, It would be used for a topping lift – that is a rope holding up the boom when the sail is lowered. These are normal on a yacht, but not used very often on dinghies. The topping lift would go to a block on the mast and then down to a cleat at the base of the mast.
The topping lift would take the weight of the boom when the sail is down and can be adjusted so the boom is above head height, it is usually slackened off when the sail is fully hoisted.
This is on a swivel so the sail can be rolled around the boom to reef it – without detaching the main-sheet (or topping lift).
You are are correct in thinking it is not used to fix the sail, the slot/cutouts just visible in your photo are used for that – to secure a small line from the sail “clew” by taking 2 or 3 turns.
Hope that helps