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 Loyal Badger colorway with an unnamed chart soon to be released with my Most Basic Mitts pattern. (*coming soon, if you'd like to make your own)
Fingerless mitts made in one size fits many. I wear typically medium/small gloves women's size 7.5 and these are pictured on my hand.