What is the main purpose of a repeater?

The primary purpose of a repeater is to extend the communication range of portable (handheld) and mobile stations. Because these radios have low output power and low-profile antennas, their direct 'simplex' range is severely limited by terrain, buildings, and the curvature of the Earth. To solve this, a repeater is typically installed on a high elevation, such as a tall building, a water tower, or a mountain. It uses high-power equipment and high-gain antennas to receive a weak signal from a portable radio on one frequency and simultaneously retransmit it at higher power on another frequency, covering a much wider geographical area. While repeaters can certainly be used to host weather nets, provide telephone patches (autopatch), or broadcast local bulletins, these are just secondary features or specific use cases. The fundamental, defining purpose of the hardware itself is to bridge the distance for low-power stations.