Most fly reels available today will come equipped with a disc drag system. Typically, this drag setup will consist of either an exposed ring of cork mounted on the inside of the reel frame, or, more commonly, a synthetic/carbon fiber drag stack inside of a protective housing which will also be mounted on the inside frame. Both of these setups are so prevalent because they work well and allow you to adjust the drag tension through a range of settings.
I wouldn't call the above systems especially complex, but a fly reel with a click-pawl drag is truly the ultimate in simplicity. With this system, drag tension is provided by a clicker (aka pawl) that ticks against the teeth of a gear. Though not nearly as powerful, adjustable, or common, a click-pawl can meet the needs of many fly anglers, mostly in the freshwater realm.
With disc drags being used on so many reels, why would manufacturers even bother with producing these seemingly ancient click-pawl reels? Here's a few things to consider to see if one is right for you!
Simplicity: Click-pawl reels are simple and reliable. A disc drag inside of a protective housing may seem totally impervious to issues, but not all of these housings are 100% sealed and water/grit can find its way inside which hurts drag smoothness. Even if foreign matter affects a click-pawl system (I have yet to have this happen), cleaning one out is quicker and easier since this super-simplistic system is totally exposed.
Weight: With such a small, simple drag system, click-pawl reels can be very lightweight and are great when paired up with similarly featherweight rods. Let's look at the weights of two popular 4/5 click-pawl reels offered right now: the die-cast Redington ZERO weighs just 3 ounces, while the fully-machined Ross Colorado LT is about the same at 2.98 ounces. By comparison, here's the weights of a few random 4/5 disc-drag reels: Ross Animas 4.1 ounces; Redington BEHEMOTH 5.2 ounces; Abel SDF 5.02 ounces; Nautilus XM 4.1 ounces.
Now, that's not to say you can't get a super lightweight reel that's NOT a click-pawl. However, the only two I can think of that come close and are comparable to the above examples would be the Waterworks-Lamson Force SL Series II 5/6 which is just 3.31 ounces, and the 3-TAND TF-40 (3–5 lines) at 3.5 ounces.
Drag Performance: One big downside of a click-pawl reel is drag adjustability. Some of these reels have tiny drag dials on the backs of their frames, but with those reels I've had my hands on, turning the knob seems to do little or nothing to change the actual pressure. Other click-pawl reels have no dial at all. Either way, the best way to apply added pressure when a fish is hooked is with your fingertips or palm along the edge of the spool. Not everybody requires a ton of drag, and if you're fishing for smaller species like trout, bass, or panfish, the amount of pressure naturally provided by the click-pawl plus any added help from your hand is likely all you'll ever need. Additionally, the click-pawl drags I've used have been very consistent right out of the box, but line typically doesn't flow off as silky as a good disc-drag due to the feeling from the gear teeth clicking with the pawl.
So, why on earth would this be desirable? I look at it the same way as some folks view a manual transmission on a car....it's just a more raw, personal, connected experience. Couple that with the fact that most click-pawl reels give off a loud, sweet tone when a fish takes line, and it just makes for a totally different feel that's a lot of fun! In fact, I'd say the sound alone is why many anglers choose one. If you want a classic, nostalgic type of feel to your tackle, this is the reel for you.
Price: Click-pawl reels are simpler and therefore usually not super expensive. Let's do another comparison using the brand Abel. Known as one of the most premium fly reel names in the world, Abel's offerings don't come cheap in any form, but you can keep the price down by going the click-pawl route. For example, their SDF 4/5 disc-drag reel comes in at a stout $695. In contrast, their click-pawl TR 4/5 reel is quite a bit cheaper at $395. If you're on a budget but realllllllly want an Abel, opting for the TR would allow you to get into the brand at a much lower price thanks to the simpler design.