The vault behind the ear is opened for a mastoidectomy - removal of part of the mastoid bone to drain infection of the mastoid air cells (mastoiditis).
The mastoid is a very interesting part of the anatomy of the skull, though I may be a bit biased; at age 7, I had a bilateral mastoidectomy. In today’s era of antibiotics and other interventions to care for pediatric otitis media and other ear problems, having a single mastoidectomy called for is very uncommon, and a bilateral mastoidectomy (on both sides of the head) pretty much unheard of.
Diseases of the Ear. Edward Bradford Densch, 1894.