Continuous control instead of switching. The spool positions anywhere across its range in proportion to the signal, so flow, pressure or direction varies smoothly. Actuators accelerate and decelerate rather than starting and stopping abruptly, which removes shock loading and pressure spikes.

Class rather than quality. Proportional valves are robust industrial components with modest response and some hysteresis, suiting most variable control. Servo valves use a different pilot mechanism for far higher response and precision, at much higher cost and with far stricter cleanliness requirements.

No - proportional valves are widely used open loop, simply to make motion smoother and more controllable. Closed loop requires a sensor and a controller in addition, and gives positioning or force control. The two are separate decisions and open loop is often sufficient.

Because the valve needs shaped current, not just a voltage - dither to defeat static friction, ramps to limit acceleration, and deadband compensation to jump the spool overlap. Without correct settings even an excellent valve behaves badly, which is why valve and card are specified together.

Cleaner than for conventional valves, and considerably cleaner again for servo valves, whose pilot stages have very fine passages. Meeting the stated cleanliness code is a requirement rather than a target, and contamination is the leading cause of proportional and servo valve failure.

The difference in spool position between approaching a command from above and from below, caused mainly by friction. It matters wherever fine positioning is required, since the actuator ends up in a slightly different place depending on approach direction. Dither reduces it and servo valves have far less of it.

Often, provided the manifold or subplate pattern matches and the electronics can be accommodated. The larger questions are usually fluid cleanliness, which may need the filtration upgraded, and whether the control system can generate the command signal the valve expects.