Letter carriers, especially those in large towns and cities, often had more mail to deliver in a day than they could carry. They began to use a variety of carts to carry mail along their routes. Two, three, and four-wheeled carts helped hold that additional mail and allowed carriers to finish their routes more quickly. Some carriers even decided to put themselves on wheels and used bicycles to make their daily rounds.
To ensure that carriers did not have to return to their post offices several times a day to retrieve more mail, some postmasters used mail distribution boxes to store excess mail. Carriers returned periodically to these boxes for more mail before continuing their routes.
Nancy A. Pope, National Postal Museum