I saved this article from a couple years ago.
Fluorocarbon fishing lines are based on a different formula than fluorocarbon leaders that makes them conducive for use on reels. Hence, they are suppler and less prone to memory, characteristics similar to a nylon monofilament. A disadvantage of pure fluorocarbon is its stiffness and memory, which make it impractical to use as a primary fishing line. It simply won’t handle or cast as well as a nylon monofilament or fluorocarbon fishing line. On the other hand, because fluorocarbon fishing lines are softer and more flexible than pure-fluorocarbon leader material, they’re not as durable
As a leader material, pure fluorocarbon offers several advantages over nylon monofilament. Manufactured from extruded polyvinylidene fluoride, fluorocarbon’s number-one selling point is that its refractive index — the degree to which light bends, or refracts, as it passes through a substance — is very similar to that of water. That makes it more difficult for fish to see. For example, some fluorocarbon formulae have a refractive index as low as 1.42, with water registering around 1.3. By comparison, the refractive index of nylon monofilament is around 1.52.
Also, depending upon the brand, fluorocarbon has a diameter that’s generally smaller than that of nylon mono of the same breaking strength, which makes it less noticeable. In addition to low visibility, fluorocarbon has very little stretch and a hard, smooth finish that is extremely abrasion resistant. Less stretch means more sensitivity, which allows you to detect subtle bites and “feel” the performance of baits or lures, including any contact they make with structure or the bottom. Low stretch also promotes solid hook-sets. And fluorocarbon’s tough finish stands up better to the abuse of structure and the raspy jaws and scales of certain game fish.