More can always be written, but you may be interested in this previous answer of mine. Popcorn was a holdover from Vaudeville, as it was a popular street food sold from "popcorn wagons" in urban areas, and there were no particular rules about bringing outside food into theaters in the early days of either Vaudeville or the movies. Popcorn wagons would often park themselves in front of Vaudeville theaters, and then later movie theaters, where sales were good. The association between popcorn and entertainment has been strong ever since. Eventually, theater-owners started to sell the snack themselves and forbade outside concessions from being brought into the theater, to supplement their own income.