Tracking targets during smooth panning motions becomes visually easier. It reduces the "teleporting" visual effect of enemies when you are spinning quickly.
If your computer struggles to maintain a consistent framerate, a subtle motion blur can mask micro-stutters, making 60 FPS look closer to 120 FPS.
Because motion blur requires the GPU to remember the previous frame (Frame Buffer) and mix it with the current one, there is a performance cost.