Leader material, as stated, is intended to spread the load over a much shorter length, be it mono or fluorocarbon. As such, it is finished harder, has a much more consistent diameter, and more prone to heat warp. The latter is primarily related to fluorocarbon.
All fluorocarbon material comes from leaders. We didn't have a reliable wind-on material until very recently. It was invented in the 70's, but we didn't begin to see it in use as leader material until the 90's. It wasn't until the early 2000's that there were mainline fluorocarbon lines available on a great scale.
One of the greatest advantages of leader material is it has highly increased knot strength for terminal connection as well as for tag to tag connections. A 20# leader on 30# braid will generally break at the lower terminal knot with true leader material. With fluoro mainline, it will generally be awfully close to 50/50 as to whether it breaks at the terminal or tag. Inline connections are easier with leader material, as well, primarily because of its harder finish. It doesn't have the tendency to roll on itself.
There are several very good leader materials out there now, available to us on the market. As much as I don't like to give Berkley any credit for their fluoro, their leader is excellent, and priced right. Rio makes excellent fluoro leader material on the fly fishing side that transfers over to our purposes very well on the finesse side. Seaguar makes my favorite, formerly in saltwater only, but now available in a slightly softer finish in the STS material that matches up really well as far as diameters go. I use the 15 and 25 for flipping and those situations I need to fish braid but want the abrasion resistance, and on finesse stuff where I need the casting distance and, I hate to say it- feel, I fish the 8 and sometimes ten.
Sorry for the long winded essay, but that's what I'm good at.