I've been reading on this forum and hear alot of you that use leaders with two different types of line.  Can someone explain why you do this and what are the benefits.   Also which two lines do you use.  When I buy my first baitcaster I think I'm gonna learn on mono line so not sure what goes with that, or even if I need a leader without knowing what the purpose is.

Simple example: Braid for mainline, fluorocarbon for leader.

 

The advantage is very strong, manageable and long lasting braided line connected 

to nearly invisible, highly abrassion resistant fluorocarbon line. Also, when you need

to break-off the florocarbon should break when braid would not.

A leader is used when conditions call for a blend of line characteristics. Roadwarriors example is a common one. For mono, about the only time you'd run a leader would be if pike are present and likely to bite the line off. In this case you'd use heavy (as in 50-100#) fluoro or wire leader. If you fish a Carolina Rig you have main line, slip weight, bead, swivel, leader and hook. In this case the leader may be the same line or one size down. This is so if you break off all you lost is a hook.

 As RW said it is what I use for my "fuse" it's my safety link. I run the sive Mono, or Fuoro, that I want to set my breaking point at. I never run the same weight leader as I run for mainline, I also never run heavier leader than my rod is rated for in lbs. test. I always use a leader. For toothy critters,,,,,,,,my custom Fluorocarbon with special features,,,,,, 100# test and 130# test.

 

For extreme toothy creatures!!