A pressure group is basically a garden pump attached to a pressure tank. This tank is filled with water and is under pressure. When water is pumped from the tank, the pressure in the pressure tank will drop and new water is automatically supplied.
Because water only needs to be supplied when the pressure drops, this type of pump is not on continuously. This can save a lot of energy compared to an ordinary garden pump without a pressure tank, which runs continuously.
Thereby, a pressure group can also be used to improve insufficient water pressure in pipes. Connecting a pressure tank to the pipe automatically increases the pressure. That just makes that shower a lot more comfortable!