The Petzl STRING serves two functions: it holds the rope end carabiner in place on a quickdraw and it protects a relatively high wear area of the sling. Available in two sizes for webbing between 15 and 30 mm wide.
I bought those because I got a lot of bent and straight gate with ample choice of slings (from the old days when quick draw did not existed and bolt were more like piton form). It works very well.
These things keep your draws in place whenever you are clipping. If you're like me and have draws without rubber stoppers (i.e. Metolius Inferno Wiregates) then you will be glad to own these suckers. They stop your biners from spinning, especially when you're pumped and trying to clip in!