Only 2 reasons I can think of:
1) Need to turn smoothly at very low speeds. Microsteps become individual steps (step, stop, step, stop) when the speed drops below about 100 steps per second. At microsteps/step, this is about 3 RPM. At 128 microsteps/step, this speed is 12.8 times lower or about a 1/4 RPM.
2) The motor is being run closed-loop. The motor microstep resolution must then exceed the encoder resolution by a significant amount to give stable operation.