Fly fishing leaders are as varied as the fish they're used to target. Ask a hundred fly anglers what type of leader they prefer and the results will likely be equally as varied. Whether you tie them yourself or like the knotless ready-made varieties, both leader types are plenty effective. There are some pros and cons to either choice, so let's quickly roll through a few of the good and the not-so-good so you can decide for yourself!
Convenience- Since knotless leaders are already tapered and ready to go, they save valuable time and require very little room for storage. Simply buy an appropriate knotless tapered leader, attach it to the fly line, tie on your fly, and you're done! If the leader somehow gets lost or messed up, just bust out another leader package and you'll be back in business much quicker than having to reconstruct the whole thing. You'll probably still want to carry an extra tippet spool, however. The tippet portion will get shorter over time as flies are switched out and frayed pieces are cut off. Carrying extra tippet material allows you to replace just the tippet portion once it becomes too short.
No Knots- The lack of knots with a knotless leader is desirable for two reasons: the leader can be stripped through the guides relatively smoothly (aside from the nail knot or loop that attaches the leader butt to the fly line), and debris won't collect so easily. Little chunks of moss and other muck loves to grab onto leader knots and it can be a real pain to remove these bits.
Cost- This is the primary reason why I don't use knotless pre-made fly fishing leaders all that often. At costs ranging from roughly four bucks on up, that can get rather pricey. The life of a knotless leader can be prolonged by simply replacing the tippet portion as mentioned above, but as you know a leader isn't going to last forever. It takes extra time, but I like the flexibility and cost-effectiveness of doing it myself. The initial investment of purchasing various line spools to make leaders might be a bit of a hit on the wallet, but you may be able to construct dozens or even a couple hundred leaders depending on the spool sizes.
Customization- Granted, there are various knotless pre-made leader styles available for different scenarios. From long, supple leaders for dry fly presentations, to shorter and more powerful tapers to turn over bulked-up bass bugs, all the bases are pretty much covered. However, I really like to tie my own exactly how I want them to optimize turnover and performance. Sometimes, I even take the lazy route and don't even taper the leader that much if I feel like the fishing I'm doing doesn't necessitate it. It could also get dang pricey having to buy different pre-made leaders if fishing in varied conditions or targeting a broad range of species.
TIP- Want the convenience of a knotless pre-made leader with the frugality of building your own? Try setting aside some time and tying up a bunch of leaders exactly how you want them, then storing each one in a small baggie. They take up very little room in a vest or pack and will be a huge time saver on the water if the need arises. The leaders obviously won't be knot-free, but that's a minor issue much of the time!