Despite many claims to the contrary servo motors do not necessarily provide the best performance on every application.
The structure of a stepper motor means that its positions are made up of discrete steps in its revolution cycle.
If an application requires very good stand-still stability, like the positioning of a vision system for instance, a stepper may be better than a servo.
As servo motors can react to changing loads and their position loop is closed via the implemented feedback device, they can oscillate at standstill, whereas stepper motors are very stable.
A note of caution
Control of the stepper motor is typically using a stepper drive, but it should be noted that these can vary in sophistication. Very common nowadays are micro-stepping drives, which can deliver in excess of 1000 steps per revolution. However some drives, although capable of a high number of steps, may not be able to position to individual micro-steps and torque between full steps may be reduced.