The anterior side of the coccyx serves for the attachment of a group of muscles.

http://en.wikipedia.org/wiki/Coccyx

The anterior side of the coccyx serves for the attachment of a group of muscles important for many functions of the pelvic floor (i.e., defecation, continence, etc.): the levator ani muscle, which include coccygeus, iliococcygeus, and pubococcygeus.