This is great for emergency and rescue situations whether you need to get someone out of a crevasse or just need to ascend a rope for a short distance. It is so light that you barely notice it when clipped to the side of your harness. Many people think of it as a prusik replacement. It is about the same weight as a prusik but less bulky. It works smoother and faster than a prusik and does a better job grabbing icy and wet ropes than a prusik.
This works great for ascending short distances but in no way is a replacement for a handled ascender like the Petzl Ascension. It it not as fast or smooth to operate and takes some getting used to. This fact is obvious to most people, but I mention it here just to make sure you understand its capabilities and limitations before buying one.
For emergency rescue situations where you need to raise an injured climber. Also can save you if you rappel off in the wrong direction and need to ascend a rope to get to the anchor.
At $34 each or $68 a pair (usually bought in pairs) it's really hard to justify the price for this. It's just a single piece of metal! By comparison, a few feet for 5-mm-7mm accessory cord to make a prusik will cost just a few bucks.