In ancient times, vestibules served an engineering function; in more modern architecture, they retain heat and reduce noise from frequently used entrances. Many American houses prior to the 1950s had vestibules, serving as airlocks to keep heat inside the home. Vestibules are also sometimes used for storage and hygiene.
A vestibule is a small room just beyond the entrance to a building. They are often separated from the rest of the building by an inner door. The vestibules of ancient Greek and Roman public buildings were the result of the extra row of columns needed to support the main entranceways. Using stone and columns, the buildings would be unstable without the small room near the door.
Vestibules in Medieval churches were designed to reduce noise from congregants coming and going out of the building. They often contained holy water founts so congregants could bless themselves before entering the church proper. Modern church vestibules have book racks and bulletin boards, supplying congregants with useful information prior to going to Mass. In addition to reducing noise, they help congregants prepare themselves for worship.
Vestibules in private residences are generally designed to keep heat in. They often have closets or space for people who are entering the house to place their umbrellas, shoes, or coats. This serves a hygienic purpose, as it keeps the rest of the interior less messy.