The M4 Messenger droid was designed to make a delivery to a specific place or individual. It could carry a message (up to five-minute long holorecordings), data, or a small object that it could deliver in a detachable, spring-loaded compartment. It usually had a small blaster for use in self-defense, but this was often removed due to local restrictions.
The M4 was not round like many hovering droids, and its slim profile and lack of protruding antennae meant it could slip into small areas. They were notorious for making deliveries no matter how much time had passed or how much damage the droid had received.