I first started using a snell knot about 22 years ago back in my teenage trout fishing days. Back then, I spent literally countless hours bait-fishing on urban lakes for planted rainbows. It wasn't the most glamorous fishing, but it was lots of fun and I picked up many technical tricks and rigging methods for catching these fish. Although I can't exactly prove that it helped me hook more fish, I began using a simple snell knot just about exclusively on my specialized homemade trout rigs.
Granted, at the time my main reason for using the snell knot was because it just looked super clean, but there are some real advantages to the knot. The biggest advantage of the knot is that it provides a straight pull since it solidly attaches to the shank of the hook rather than the eye where it may slide around.
There's some variations to the snell knot, but this simple version is the one I learned many years ago...it's worked well and is really easy!
1.) Bring the line through the hook eye (I'm improvising here!) while making sure you have enough of a tag end to work with.
2.) Make a loop in the line while leaving some space between it and the hook eye.
3.) Wrap the tag end about 5 times (going towards the eye) around both the line and hook shank. Make sure the rear loop stays open the entire time. For this example, I go around fewer times since I'm working with thick rope.
4.) After the last wrap is completed, bring the tag end back through the open loop.
5.) Now, slowly snug the knot by working the main and tag lines. If needed, you can slide the knot forward so it's right up against the hook eye before FULLY tightening the knot.
While it may not be a super-common knot in fly fishing, it can be used with certain patterns—most notably saltwater patterns like some tarpon flies that are tied with a portion of the shank left bare behind the eye. It's also handy for tying two hooks together for a big tandem-hooked pattern or even for using two flies at once. Simply leave a long tag end, and use that to tie on another trailing fly. For those of us that conventional fish too, bass anglers use the snell knot for techniques like flipping, punching, and drop-shotting.