It's basically needed to match throttle angle with engine output power.
On some turbofans you have one or two rigging holes (mostly part power and flight idle) that have to match certain EPR criteria. I recall doing part power adjustments as this: insert a 94 degree part power stop in the rigging hole, start engine and advance thrust till you hit the stop, adjust a trimmer on the engine while its running to match a certain EPR e.g. 1 click with an allan key in the trimmer was 0.001 EPR or something. My memory fails. I did this on old RB211C's on 757 and JT8's on the 727.