Additive disturbance in block diagram representation
The pump keeps the tubing pressurized, the desired input flow is tuned via the valve. However the net flow rate that feeds the tank is not this inflow, it is the difference between the inflow and outflow.
The outflow caused by the pump is modeled as a disturbance because, in the current example, the goal is to model the liquid's level inside the tank. The outflow maybe considered a byproduct of the pump's operation.
Although the above diagram describes the plant (the physical process of the hydraulic system), if the control of the plant was studied later a possible design goal could be to maintain a constant and controllable level of the liquid in the tank (this requires feedback control aka a closed loop) and/or reaching the steady state value(desirable level in the tank) fast.
In addition a pump's flow rate tends to decrease as the pressure inside the tubing increases.
Think about a garden's hose, which sinks water into a flower pot at some flow rate and the water drains out of the flower pot through the bottom. Then if the inflow is much larger than the outflow the soil's surface will be flooded not being able to properly absorb the incoming flow of water. On the other hand, if the outflow is close enough to the inflow then the soil will be able to absorb the incoming water flow, not resulting in the flower pot's water overflow.
The block $G_t(s)$ describes the tank's response therefore the inflow-outflow difference is its input. The system $G_v(s)$ is the valve's response to the user input.