This practice is controversial and considered inhumane by many animal welfare organizations. It's done for purely aesthetic reasons and offers no health benefit to the dog. Cropping can also cause pain, infection, and behavioral problems.
In many countries, including the UK and Australia, ear cropping is illegal.
While you may see many American Dobermans with cropped ears, it's important to understand that it's a human-imposed alteration, not a natural trait of the breed.