Q. Why do bass fisherman use leaders?
For example, the reel's full of braid and you have a fluorocarbon leader.
A. Specific to your example above, the reason some one would tie a fluorocarbon leader to braid is purely for the visibility or actually invisibility of the fluoro vs the braid.
Braid is super strong and will hold up really well for more uses, but it is very visible both above and underwater.
Fluorocarbon on the other hand is nearly invisible underwater. Therefore chances of a cautious fish taking your lure are greater with fluoro and then chances of you horsing in that heavy weight are good with the braid.
Sometimes the stronger line will be used as a leader also.
In the case that a reel is spooled with mono-filament line and then has a fluorocarbon leader, the stronger leader is being used to absorb the shock of the strike and give a better chance of landing that big catch.
Hope this answers your question.
Thanks for stopping by.
-RR