What is the function of the pump?

Answer 1

The purpose of the jockey pump is to keep the water pressure in the pipes within a specific range when there's not a fire, so that the sprinklers won't go off randomly. Since pipes leak, over time, the water pressure inside them automatically goes down. The jockey pump senses this, and then fills them back up to normal pressure. If a fire happens and the pressure drops dramatically, the jockey pump won't be able to keep up, and the drop in pressure will trigger the large fire pump to start sending water.

Secondarily, this pump prevents sprinkler systems from being damaged when the fire pump begins sending water. If a system does not have a jockey pump keeping it pressurized, it may have a relatively low pressure. When the fire pump starts sending highly pressurized water through the pipes, the sudden change in pressure can damage or destroy the system.