They are often called fast snaps. If you tie your own knotted leaders, they may come in handy if you use one end of a blood knot as a dropper. The end would be too small to change flies more than once or twice. Using the clip will allow you to change the fly on the dropper as often as necessary. I would only use them for nymphs. They are handy for beginners who are not very good with knots and/or if they will change flies often. As stated in a previous post, it is sometimes a challenge to remove a fly.