Sucking beams are used by for hauling various cargo that is either too big, or too hazardous to be brought inside a space-ship. Additionally, they have proven useful in combat as a final measure, or as secondary tool.
An active sucking beam can use up much of a ship's energy if used in unison with other systems, especially when the ship is traveling at high speeds.
P-Fleet Constipation-class cruisers are outfitted with two sucking beams - one in the front, and one in the back. It is unknown if the same applies to other classes of P-Fleet ships.