The control valve on the discharge can be divided into two kinds:

Is a kind of switch control: either fully open or close, traffic or maximum or minimum, no intermediate state.

such as ordinary electromagnetic pass valve, electromagnetic reversing valves, electro-hydraulic directional control valve.

The other is a continuous control: valve port can according to need to open any one of the opening, which control the size of the through traffic.

this kind of valve with manual control, such as throttle valve, also have electric control, such as proportional valve, servo valve.

So the purpose of using proportional valve and servo valve is: electric control way of flow throttle control (of course after a change on the structure can also be used to realize pressure control, etc.)

Since is the throttle control, there must be energy loss, servo valves and other valves, its energy loss bigger, because it needs certain traffic to sustain prestage control oil circuit work.