Loop knots have an integral place in the way most experienced anglers rig their flies. Despite common misconception, there are a great many loop knots that are exceedingly simple to tie. Sure, there are loop knots that are cumbersome or even difficult to tie, but some of the best loop knots are easy to learn and to tie streamside. But the main reason that most experienced anglers readily use loop knots is that, in a great many scenarios, loop knots provide significant advantages over knots that are fixed to the fly's hook eye.
Following are a few reasons why, if you're not already regularly using loop knots in your fishing, you should be. Even beginner anglers will likely be familiar with the most common of these reasons, but the others may be new information even to some of you who've been at it for a while.
Better / More Natural Fly Movement
This is the justification for loop knots that most anglers learn first and usually in the context of streamers or flies like the wooly bugger. Flies that are supposed to represent larger, swimming insects in the water have a significantly more natural movement when attached to your leader or tippet with a loop knot. Instead of being attached with a rigid, fixed connection that restricts the fly's movement, a fly on a loop knot can slide and float more freely, producing a more natural action.
But this doesn't apply to only streamers. I often encounter anglers that have worked loop knots into their repertoire when fishing streamers, but limit their use to those scenarios. The same principle that applies with streamers also applies to wet flies and most certainly applies to dead drifting nymphs.
A more natural movement of the fly results in a greater number of takes and that results in a higher catch rate. Simple.
Sink Your Fly Faster
In his book, Fly Fish Better: Practical Advice on Tackle, Methods, and Flies, noted angler and author Art Scheck highlights a lesser known reason for choosing a loop knot for your terminal connection: faster fly sink rates. The logic behind Scheck's contention is that a weighted fly (tungsten bead head, lead bead chain eyes, etc) can adopt a nose-down position and sink faster when its connection to the leader or tippet is a flexible one. According to Scheck, though this effect is more noticeable in still water it also offers an advantage when using a strip-pause retrieve, allowing the fly to sink more readily during the pause.
Though the advantages loop knots offer to flies that need to get deep may not be pronounced, it offers anglers another reason to choose a loop knot when rigging their flies especially when considering that many of the situations described above in which the angler is seeking freer, more natural movement for their flies are also situations during which the angler is hoping to get their fly down deep, often quickly.
Break Fewer Knots
While all specific claims about knot strength and breakage rates should be taken with an enormous grain of salt, it is valuable to consider that some loop knots are widely held to be stronger than their fixed-connection counterparts. My personal favorite loop knot, the Non-slip Mono Loop knot, is considered by many anglers to have 100% knot strength. While remembering what I just said about treating strength rate claims and such as considerably dubious, keep in mind that means many experienced anglers are suggesting that tying the non-slip loop will in no way degrade the strength of your leader or tippet. You'll have a hard time finding people to make that claim about fixed knots such as the clinch knot, improved clinch knot or even the Orvis knot.