I thought that the purpose of mirrors in gyms was to check your form.

Used to be, people relied on spotters, instructors, coaches and workout partners to check their form. The way it's still done in old-style boxing gyms.

Similar thing used to be the case when it came to dressing, grooming and bathing. In the old days, people relied on servants such a valets or ladies maids or family members to assist them with these tasks and tell them they looked good, and they often groomed and bathed in social settings - as in communal baths and the tradition of the "reception toilet," similar to going to a beauty salon or barber shop today only it was done in people's homes.

Even after glass mirrors were invented in the Renaissance, for centuries they remained precious, extremely costly and tiny - only big enough to show the face/head, or part of it; and coz indoor lighting was dim, people mainly relied on other people to serve as their mirrors. Also, a lot of clothing - especially the clothing worn by well-off women - customarily required help getting into and lacing or buttoning up the back, so dressing alone like we often do today wasn't an option.