Because it will respond rapidly to change, limited only by the bandwidth of the opamp and power transistor(s). Using a software feedback loop will be slower - this may or may not matter, it depends what you're testing with the load.
Because it will respond rapidly to change, limited only by the bandwidth of the opamp and power transistor(s). Using a software feedback loop will be slower - this may or may not matter, it depends what you're testing with the load.