In the salt however, there's a different reason for using them. Of course it's nice to be able to change the hook if it hits a stone in back cast, but more importantly, I like changing weight on the go.
I have a small "box of tricks" (I wrote something about it a while ago, but I can't find it again :-). It contains different "add-ons" for flies and tubeflies. I have, amongst many different bits, tungstens beads lines with tubing to protect the leader.
With these, I can change the weight and hence fishing depth of a fly as fast as it takes to clip off the hook, slide bead onto the leader in front of the fly, re-thread the tube and tie on the hook again.
This picture was snapped as I was fishing a drop-off in a small bay. Five steps forward would see me with water in my waders. On the other side of the dam you see in the background, the water is very shallow.
The bead is heavy enough that I can count the fly down to at least 6 feet, and more of less keep it there on a long cast, as long as I don't retrieve too fast. I often change shooting head to change fishing depth as well, but if it's just for 25 casts, I really like this method - especially on tubes as the knot between hook and leader is at the back of the tube and thus protected from the heavy bead, contrary to the "normal" hook fly.
With a selection of tungsten- and brass beads in different sizes, you can adjust to everything from 5 inches of water (unweighted tube) to 6-7 feet or there about and everything in between.
Have a great weekend!