#1 - I usually use half a spool of leadcore per reel, so I'll fill 2 reels for the price of one. I put fly fishing type loop-to-loop connections [sized for 10-wt lines] on both sides of the leadcore, so I can easily add or replace the backing and/or the leader.
#2 - It fills up the reel, so there's less 'coil' imparted to the leadcore.
#3 - Backing gives me ample line to fight big stripers with !