I don't remember exactly what I bought this for, but I got it because I have the Domke F-803 Camera Satchel, also in sand, which I use as a general shoulder bag (with laptop, papers, etc.) This pouch mates perfectly to the end of the F-803. About the same height, and same color obviously.
Now, I can carry a camera with me as well as fill up the 803 with papers or whatever. The thing about the larger Domke bags (and this style of bag in general) is that while they may look like regular shoulder bags that will hold all kinds of stuff in addition to your camera gear, what really ends up happening is that your camera gear's awkward shapes use up all the usable room in the bag. You might have plenty of room left, but odd spaces not usable for papers, laptops, etc.
This is particularly frustrating if you use a bag like the 803 as a carry-on for airplane travel, because there isn't room for all the other stuff you want to bring with you.
Of course, this is a rather small pouch, so you're not going to fit a full size DSLR in here. However, I've been frustrated traveling with my Canon 40D and two or three large lenses, so I got an Olympus EP-2. This bag is basically perfect for it. I have two lenses for it, the kit lens and the Panasonic 20mm f/1.7, which is what is normally always attached. Both lenses, extra batteries, and a small clamp camera mount all fit in here. It's not quite big enough that you can just toss the camera in, though - things have to be arranged properly for it to fit. That's not a big deal though; you usually have to pay attention to how things are arranged in camera bags anyway.
So the point is, if you've got an F-803 and an EP-2, you like to use the 803 as a shoulder bag for carrying stuff besides your camera, and you like to use it as a carry-on, this is ideal. A lot of conditions there, but you can extrapolate it to whatever equipment you happen to have :)
The greatest advantage is that the camera is right at hand. The EP-2 and accessories will fit in the front pockets of the 803, so if you don't have other stuff you like to carry in those (both pockets on mine tend to fill up with all kinds of other useful stuff if I'm traveling or generally using the bag) you don't really need this pouch. But you'll never get stuck opening all of the clasps and compartments (which is often difficult to do quickly on Domke bags) fumbling for your camera again, because it's very easily accessible this way. If you put this setup under the seat in front of you on an airplane (or wherever) and face the pouch toward you, you can just reach down and take out the camera very quickly.
Of course, that means it's easy for someone *else* to grab it quickly, so if that concerns you, either learn to keep track of your bag and/or use a different arrangement :)
Finally I'll mention the attachment details. It's got both velcro straps and a belt loop. You could theoretically attach it to your belt, if you're not fashion-conscious. The 803 and some other Domke bags have a clip at the end that's designed to go through the belt loop of this pouch (and other Domke accessories I presume). Not all of the bags do, though. The larger Domke bags do all have "handles" at the ends made of the same material as the shoulder strap, and the velcro attachments on this pouch are designed to attach to those. The velcro should be quite secure, but I trust the clip through the belt loop much more.