Direction and rate together. The spool shifts progressively rather than snapping over, so its position determines both which way flow goes and how much passes. One valve replaces a directional valve plus a separate flow control, and does it with continuous rather than fixed settings.

Motion quality. Actuators ramp up to speed and ease off at the end instead of starting and stopping abruptly, which reduces shock loading on the structure, removes pressure spikes that shorten hose and seal life, and makes the machine much more controllable for a skilled operator.

The spool's flow characteristic determines how flow varies with position, and therefore the feel of the machine. A spool giving fine control near neutral behaves quite differently from one optimised for maximum flow. Changing how a machine feels is often a spool change rather than a new valve.

The dead region around centre where the spool has moved but no flow passes yet. It ensures the actuator holds still at neutral, but it also means small commands do nothing until the overlap is crossed - which is why the amplifier's deadband compensation exists.

Onboard is now the norm and generally preferable: no wiring between card and solenoid, the card is matched to the valve, and commissioning is a single item. Separate cards suit installations where the electronics must sit in a protected cabinet away from the machine environment.

Only as well as any spool valve, which is to say not reliably over time - there is leakage past the spool by design. Any load that must not creep needs a dedicated holding valve on the cylinder, regardless of how good the directional valve is.

Usually the electronics rather than the valve: dither set wrongly, no deadband compensation, or ramps too short. Contamination causing the spool to stick is the other common cause. Both are worth checking before concluding the valve is faulty. A card setting costs nothing to change; a replacement valve costs a great deal.