Why are fluids put under pressure?
When a pressure is applied on a fluid, which can be either a liquid or a gas, the pressure is spread out uniformly and the same pressure exists at all points in the fluid.
Pressure is applied to fluids in many applications that require the movement of a load but the force cannot be applied directly to the load. Or in cases where it is easier to apply a small pressure on the fluid for a long time and have it multiplied by the fluid. Using fluids to transfer force is also often more efficient than using shafts, strings, etc.
Some applications where this is made use of include brakes, pneumatic motors, cable jetting, vacuum pumps among many others.