It isn't. The admin_background mixin is only used once at this stage, but I've separated to out as it might evolve into a reusable piece of code. It's a gradient-based implementation of the faux columns technique. We want the white body area behind the admin controls to match the height of the tabs column, but we also want both to expand as more tabs are added, so we can't just give it a fixed height. So we wrap both the tabs and the body in a div (#admin_container) with a background that's transparent on the left and white on the right, which gives the appearance of the body column stretching all the way to the bottom.

There are some nice modern techniques for getting equal-height columns without this kind of trickery, but they're not as well supported yet as gradients.