Many Wet Fly fisherman fish a cast of more than one fly, dropping one or more flies from junction knots tied directly into the leader.  When fishing this style it is important to keep the dropper hanging no more than 3 inches to prevent tangling. After changing the dropper once, maybe twice the entire rig has to be retied to facilitate new flies being added.  These leaders are designed with a tippet ring to add the dropper to easily, making removal and retying the point fly a thing of the past.  Faster fly changes means more time spent with your flies in the water.  And we all know what having the fly in the water more will lead to!  Please check with local laws before fishing with more than one fly.