Most professional computer artists also carry at least decent programming skills. It's a fundamental part of being able to use graphics (2D and 3D) packages to their fullest potential.

Sadly, the reverse usually isn't the case.

Coder graphics are used at the earliest part of a game's development, so coding can get under way while the artist(s) is still producing some rough mock-ups (and generally being lazy).

Coder graphics usually are all they need to be, rough approximations of the final object's dimensions, looking more-often-than-not suspiciously like cubes, spheres, cylinders and (depending on the package used) teapots.