Good for keeping your hands warm and your fingers free to do things. I've heard they are also good for doing fancy penwork while protecting your hands and paper.
These were made from the For The Honor Of Greyskull colorway with the Power Up Mitts pattern (soon to be released).
Fingerless mitts made in one size fits many. I wear typically medium/small women's size 7.5 gloves and these are pictured on my hand.