[Coldsaturn]

It's basically putting "joints" in the figure so that for example if you raise the wrist, the elbow and shoulder also move accordingly. Without, you would stretch the skin of the wrist with the rest of the body where you left it.
I don't think the missing rig is a matter of infold trying to protect their assets, rather than the ways people use to extract said files to begin with, which can leave you with a sort of incomplete product.
It's possible that as time passes people will fix these models, especially if more get interested in 3d renders of the boys, but we'll see.