When we receive a DC motor for service we will profile the commutator to assess the condition. The last picture shows one in an ‘as received’ condition. The differences in the segment heights is what the surface of the carbon brush sees. If not smooth, then it will wear the brush away quicker than it should. Then, you’ve got more maintenance to do than you’d want to. Or, more chance of an in-service failure happening. Imagine the difference between a cobbled street and the surface of a racetrack. That’s the best analogy I can come up with.