If you have Improved Steady Shot, casting 2 Steady Shots in succession on targets whom are immune to damage will still provide the buff. Useful for when Mages Ice block and you have nothing to do but watch them sit in it. Well, now you can get a hefty 15% haste bonus from them dillydallying in Ice block. There's many other uses, Paladin bubble, banished targets and so forth. Where the exploit comes in, is usually proc abilities don't proc off of immune targets, you don't get Master Marksman stacks from immune targets, so this is actually quite useful.