Along with other characteristics, beauty/physical appearance is often used to establish a social order. Whether that's women's beauty or scarification, status can frequently be determined by appearance. In a lot of cultures, certain characteristics are deemed particularly spiritual, in either a positive or negative sense, so it ties into religious beliefs as well (albinism in Africa and many physical deformities in India are examples of this). Then of course there's the purely biological side-- physical markers indicate good or bad breeding potential. For women, that's things like wide hips to facilitate childbirth, for men it might simply be strong muscles and a powerful build, indicating a good hunter/provider.

All of these would vary between cultures of course, but the ones I've mentioned are pretty common in various forms.