When you remove the air from a closed space, you create a pressure differential between the inside of that closed space and the outside of the space. If the enclosure is strong enough, it will stand up to the pressure differential. If it’s strong enough but there’s a simple plunger attached, the pressure differential will cause the plunger to move toward the low-pressure area until the pressures have equalized.
The same principle applies to spaces that are filled with liquids, albeit with different mechanics. Because fluid cannot be expanded or compressed the way that air can, removing the fluid from one side of an enclosed plunger-system moves the plunger in a much more stable manner; a pneumatic vacuum-plunger system will wobble back and forth slightly as the air expands and contracts back and forth until stability is achieved; a fluid system’s plunger will move once, decisively, and stop exactly where the fluid forces it to stop. That’s because there is no vacuum created; it’s merely the movement of fluid out of a container and the movement of the container to match the new amount of water inside of it.
The machine that removes the air from one side of the plunger arrangement is called a vacuum pump. Vacuum pumps are available in three grades: rough, middle, and high-vacuum. For industrial purposes, all vacuum pumps are rough pumps because it’s easier to increase work performed by increasing the surface area of the plunger than by increasing the strength of the pump.
In pneumatic systems, the fact that a stable vacuum can be created and essentially put ‘on hold’ until its work is needed is particularly valuable in that a single vacuum pump can, by the use of a pneumatic manifold or other switches, create dozens of pockets of vacuum in specially-designed cylinders all around a given device, which can then be ‘popped’ for bursts of work at a later time.
Because of the different physics, vacuum pumps cannot exist in hydraulic systems; no matter how strong the shell of your hydraulic manifold, you won’t ever be able to create a stable vacuum in fluid — the pump will overheat and burn out or the manifold will crack or crumple before that particular law of physics gives up. For pneumatic systems, however, vacuum pumps are vital and powerful units that every industry has found use for.