Long before we had inches and centimeters, we had hands. The breadth of a man's hand was the metric of choice at least as far back as ancient Egypt, and this bodily ruler is still used in a few countries, primarily to measure the height of horses.
This makes sense. As tools go, this one is, well, handy. You're not going to misplace it, and it's familiar enough that everyone knows what hand-sized means, at least roughly. And it's easier to handle than, say, a foot.