Why are pumps and valves are necessary in hydraulic systems?

The basic and most common functions of valves in a hydraulic system are to regulate the flow and pressure of liquids.

There are varieties of valves as per their functions such as:

Isolation valves, to open and close the system,

Flow control valves/Balancing valves, to co control the quantity of flowing liquid,

Pressure relief valve, to release the excess pressure from the system, etc.