There are legitimate differences in line tapers, and that the more you learn about them and match the right line to your own style and interests, the better you will cast and the better you will fish.
When I look at the amount of time and energy devoted on this very site (with good reason) to topics related to cartridges for rifles, or shells for shotguns, or arrows for bows, I cannot help but wonder why so few fly anglers carefully consider the line they use. Seriously, if anglers thought half as much about what line to use on their 5-weight as a shooter does about whether to fire 165-grain or 180-grain bullets through a 30.06, most questions about how to effectively cast would answer themselves. And a good fly line can make a budget rod dance, while crappy line will knock the performance straight out of the most expensive, next-generation graphite rod.
It’s one thing to decide whether you want a double-taper (say, for delicate dry-fly presentations) or a weight-forward line (a good all-around option), or a sink tip (for fishing streamers)—that’s the easy part. But even if you are looking at the most common line (the WF5F) it’s important to realize that all options are not alike. Different elements, such as coatings, core materials, and surface textures, all matter, and the line makers guard their manufacturing processes like you wouldn’t believe. I’ve seen rods and reels get made (and pretty much every fly fishing widget and gizmo) but I have never, ever seen the secret sauces and machines that go into making fly lines. And I doubt I ever will. It’s that big of a deal to keep those things under wraps.
That said, one aspect of fly lines always in plain sight is taper design. Fly lines are different than the monofilament, braid, or fluorocarbon used with conventional tackle in that they’re weighted. The weight of the line is what the angler leverages to hurl a very light fly, and the weight rating of a line, in general, is meant to correspond with the size rod it’s paired with. But where that weight is distributed in a line can make a huge difference when it comes to casting and fishing. For example, a line with all its weight packed near the front is going to generate energy to chuck flies, but a distance caster is going to want some weight reserved for the middle section, otherwise you’ll have to strip in all the belly, and then shoot on every cast to generate any length. Mending is critically important to an effective presentation, and some lines mend better than others. Again, if all the weight is bunched up like a bullet in the front of the line, and all you have to lift and move is running line, you won’t be able to mend as effectively as you would with some weight distributed farther back. If I’m turning over big streamers and making repeat casts at moderate or short distances (like banging the banks from a drift boat), I want that weight packed up front so I can lift and fire at will. On the other hand, double-tapers work well for dry-fly fishing with rods that are 4-weights or lighter, in particular. For most fiberglass rods, I still like weight-forward lines, but that’s just me.