As an avid conventional bass fisherman, I know the importance of choosing the right rod for the job. Those huge piles of rods and reels on the front decks of bass boats may look totally excessive to some, but each bait and technique demands a particular setup for the best efficiency. After all, it's not ideal to throw a small crankbait on a heavy jig stick, nor is it the brightest idea to flip with a puny dropshot rod. While years ago rods used to not be marketed so specifically, nowadays that's all changed. There's topwater-specific rods. Froggin' rods. Even rods to fish Senkos. There's no doubt these sticks are usually designed and tuned perfectly and do feel pretty awesome with the baits for which they were designed, but in my opinion it can be a bit much—and that's where the relation to fly lines ties in.
Those specialized bass rods are often referred to as "technique specific" rods. Fly lines are not much different, but they'll also often be marketed as species-specific as well. There's fly lines for redfish, tarpon, bass, trout, soft presentations, more aggressive presentations, etc etc etc! The list extends far beyond there. Most of these lines I've tried do their intended jobs and do them very well, but you don't always want to think of each line as only having ONE purpose. The thing here is that many of these lines work well in other scenarios outside of what they're marketed towards.
One example is an angler who enjoys fishing both saltwater and largemouth bass. Flies for both can be bulky and/or weighted. The typical saltwater line is often made with a shorter, more powerful head that's great for quick shots and has ample power to turn over these stouter flies. Well, guess what? Since bass flies can be similarly heavy and/or bulky (although some are excessively so), bass fly lines are built with a similar design. One could likely use a certain salty fly line for both saltwater and bass (or vice versa) with satisfying results—I sure do!
Pay attention to a fly line's name, specs (grain weights, etc) and description. If a picture of the line profile is shown as well, all the better. Compare head lengths, grain weights, and other attributes with other lines. You'll notice that some lines share similar descriptions and designs and can often be good choices for more than what it says on the box. Owning a line that can serve solid double-duty can save you from having to purchase yet another expensive line for other species. While getting the most specialized line for the job is likely the very best way to go, it's not always necessary for solid performance. Get familiar with the designs and strengths of your lines and you may discover they can be a lot more versatile than you first thought!