Leech- & Footlines
As a sail wears and stretches, in heavier winds the leech could flutter extensively. This could cause excess wear on the sail. If you hear the sail "machine gunning" you might want to snug up these leech lines. Same with the foot of the headsail, but probably not so noticable. I''ve also heard of some folks who tighten them up to make a "pocket" in light air. Not sure if I but into this latter theory. Might just depend on your point of sail.