A few reasons for me
I fish mainly from the bank and it's a lot easier to break off 30# braid with a 14# FC leader than it would be straight braid if I get snagged, It's a confidence thing in clear water, fishing from the bank I usually only have 2 combos with me so I retie a lot which can really eat away at my mainline if I'm not using a leader, braid isn't very abrasion resistant and the leader solves that problem.