The Directional valves in a Hydraulic System serves three purposes, check https://www.tridenthydraulics.co.uk/.
* They direct the flow of liquid in a particular direction
* Stop and proceed with the flow at anytime
* Change the direction of the flow within the circuit.
There are many check valves present within the system to check the flow of liquid in the particular direction and do not allow it to drift from the set path.