Let me be the first to admit that I spend way too much time looking at tools online.
When I found this little guy, I shamefacedly had to admit that I didn't know what it was for (and assume there are other self-taught mechanics out there who are unfamiliar with this tool too). My friend Steve, who spent a few years in his younger days as a motorcycle mechanic told me that he had one in his tool box and filled me in on what he used it for.
"On Japanese bikes," he said, "they use a lot of phillips head fasteners that can sometimes get over torqued or otherwise stuck on a part. It's easy to strip the fastener and maybe even ruin the part if you try to force it off. A smack or two with a hammer on the impact tool breaks the fastener loose so you can easily unscrew it without stripping the head."
Since I added this tool to my tool box I've needed to use it a couple of times and it works on torx and allen fasteners as well as phillips or slotted head screws (I use my 3/8 drive bits). I typically don't have a problem on the fasteners I'm regularly working with to do scheduled maintenance because they are regularly removed and re-tightened, but sometimes a seldom accessed fastener can be a little stubborn and this handy little tool makes quick work of breaking it loose so I can remove it.
There are a number of manufactures that produce these things, but the GearWrench impact driver is what I have and it seems to do the trick. If you occasionally run into a stuck fastener, this will usually make quick work of breaking it loose.
The other day I was helping a friend who was doing his own oil change for the first time. The dealer (or whoever had done the last oil change) had over-torqued several of the the torx fasteners on the derby cover of his Harley making them pretty reluctant to break free. Rather than forcing them (and running the risk of stripping them), a smack or two on the impact driver broke them loose so we could change the oil in his primary without ruining the fasteners.
It doesn't take too many situations like that to more than justify the cost of the tool.