There are several things that mummified reamins like his can tell us. Most of this comes from skeletal analysis. Bones are excellent for telling us the general size and shape of things: they don't change much after death, and nobody's taller than their skeleton. The wide hips, club foot, and overbite are all discernable from bones. Bones also record serious trauma; fractures and breaks will leave identifiable scars with which we can often determine the kind of impact and how long before death it occured.

As for details like his skintone and face, artistic liscence must taken for such CGI images, since they cannot be sufficiently constructed from bones or mummified soft tissue. While it's pretty neat to find, soft tissue can actually tell us very little.