Answer: In orthodox church it is almost a compulsory for priests. In Latin America (both today and in the past as well) it is a mark of (especially in younger guys) of being of Marxist or leftist (or "alternative") conviction. In Europe it is not like that anymore ...today it is much more question of aesthetics and fashion (in EU).
Step-by-step explanation: of course, we can take a different, unpolitical approach. We can look at these things from psychological or psychoanalytical perspective.