Pilot positioners are used for two reasons. The most common reason is to adjust the spring range, ie the pressure at which the actuator is fully open or closed. I've seen them most often on steam line setups where there is a 1/3 capacity valve for fine control and a 2/3 capacity valve.
The other reason is that you can get faster or slower movement because of the direct supply connection instead of going through a control device that might be volume limited.
I've seen people take out pilot positioners and the damper alone can't close or open a set of dampers. You can also use them to stage equipment by changing the start point and end point. For example you can put a heating valve and a cooling valve and stage them where the heat comes on from 3 to 6 psi and the cooling comes on from 8 to 11 psi, this also gives you a deadband from 6 to 8 psi where you have not heating or cooling.
Law Of The Thermostat: He who has the thermostat wins!!!!!