Snap shackles are normally put on halyards
for faster sail changes from the days of hanked on sails, no headfoils, and one jib halyard. You would have released the lower hanks on the existing sail and bend on the new sail in that area. you would then drop the sail switch the halyard to the new sail, detach the sail you just doused and set the new one. With so much to do it was necessary to have the snapshackle on the halyard. Then they went to double headfoils and single halyards so you still had to drop one sail, switch the halyard and hoist the second sail. Now we have double headfoils and double halyards so it is nowhere near as critical, but it is still convenient to use the snapshackle.There is no good reason however, why you would need one on a roller furling jib, so use a D shackle if you want to.