The double acting cylinder is used in either pneumatic or hydraulic systems. It is a good advance on the simpler Single Acting Cylinder.

It has two inlets, which means that fluid can be used both to push out the piston, and to return it, and can also be controlled to go to positions between. Because there is a volume of fluid on both the outward, and inward sides of the plunger, the cylinder will be held in place when there is no exhaust.

Although this means you need an additional line per cylinder, it also means you have greater control and stability. Lego started producing these for their pneumatic range, and they did make things somewhat easier, as you no longer needed an additional non-return valve brick, but it also used different pumps.