A few reasons I would do a leader of mono or flouro
Stealth, when situations calls for it, but can’t hurt if you’re all rigged up already.
Finesse, well the braid to finesse sort of follows the stealth reason. But I may end up doing a 4# leader to 20# braid, especially for ice fishing. Well I would’ve have a more appropriate finesse of 6# braid setup, but if ever the case of improper planning should happen.
Prolonged life, braids last pretty long, but it does wear out. Especially were the fish teeth maybe gnawing on, or just abrasion contact surfaces like zebra muscles.
Toothy leader resistance, yes I may use 30# mono flouro to a 20# braid. Maybe I just want my leader line to last longer as well.
Finesse casting distance. Well maybe a few feet difference, but when casting UL lures, any weight forwarded setup couldn’t hurt it. I noticed that when I went with just fluorocarbon lines, my casting distance fell a few feet short. Braid lines themselves given me the casting distances already, optimized is hey why not.