The leech of a mainsail gets more abuse than any other part of the sail. When the sail luffs, the Dacron leech tape snaps back and forth which, over time, breaks down the fibers in the sail at the inside edge of the leach tape. The sailmaker’s term for this damage is called “hinging.” To lessen the damage, UK Sailmakers offers the option of putting a wide strip of taffeta up the leech. This reinforces the sail and moves the hinging point forward into the sail where the sail doesn't flutter as much. The harmful effects of hinging happen faster when the leech line is too loose and the leech of the sail flaps violently for minutes at a time.
The leech of the sail is also assaulted by the sun’s harmful UV rays when it is folded on the boom, but not covered. Having a wide strip of taffeta offers some, but not total protection.
The wide strip of taffeta can protect the leech from the chafe of the boat’s topping lift that sways around loose while the boat is sailing.
All in all, a wide strip of taffeta on the leech of the mainsail is a good choice in aiding durability.