Rate alone - how much fluid passes, in proportion to an electrical signal. It does not select direction and does not set pressure. Actuator speed becomes something the control system commands directly rather than something fixed by a hand-set throttle.

Holds the pressure drop across the metering edge constant, so flow follows the command regardless of load. Without it, flow depends on the pressure difference across the valve, so the actuator slows as the load increases - the speed set is not the speed obtained.

Whenever the load varies during the movement, which covers most lifting, moving and forming applications. Only where the load is genuinely constant throughout the stroke can an uncompensated valve give repeatable speed, and constant loads are rarer than they first appear.

If direction is already handled adequately and only speed needs to become variable, a flow valve is the smaller and cheaper change. If both direction and rate need proportional control, a proportional directional valve does both in one component and is usually the better answer.

Depending on the arrangement - meter-in controls flow entering the actuator, meter-out controls flow leaving it. Meter-out is what controls an overrunning load, since restricting the outlet is what stops the load running away, and the choice affects both control quality and heat generated.

Meter-in throttles the supply to the actuator and suits resistive loads. Meter-out throttles the return and is what controls overrunning loads, because it restrains the actuator from the outlet side. Many circuits need meter-out specifically, and using meter-in there gives uncontrolled motion.

Yes - all throttling converts the pressure drop into heat, and a flow control working hard is a significant heat source. Where a large proportion of the pump's flow is being throttled away, a variable displacement pump matching flow to demand is the more efficient approach.