Called 'luff jackline'. It allows the slugs to become 'loosely attached' from the luff when the sail is dropped allowing all the slugs to 'stack up' one on top of the other in the track. Without the jackline the 'folds' of the sail make the 'stack up' too bulky to affect a (first) reef.
When the sail is dropped (sufficiently for the first reef) the jackline automatically loosens and consequently the attached slugs (attached the jackline) are no longer 'rigidly' attached to the sail's luff.
When the luff is fully tensioned the jackline also becomes tensioned thus holding the slugs in 'normal' position.
Simple speak: such makes reefing easier.