Mittens are a common knitting gift for a lot of reasons. They are relatively small (compared to, say, a sweater or a scarf), and therefore fast to complete. They have a lot of different parts, which makes it feel like the knitting is going quickly. And most importantly, everyone needs a lot of mittens. You're forever losing mittens, or getting them all gross and throwing them away.
(The consumer trend lately has been to buy teeny weeny mittens and gloves which stretch to fit, and cost only about $2 per pair. The big problem with these mittens and gloves is that they don't actually keep your hands warm!)
Mittens also provide the potential for endless variety. You can try out different stitch patterns (on the back only, please - no one likes a weird stitch on the palms of their mittens), stripes, different yarn types, colors, and construction types. Much more so than many other knitted gifts, mittens are interesting for the knitter to knit, and useful for the recipient to wear.