It's usually for all of the reasons mentioned above - you want to have control over your depth of field because you're shooting a landscape, or you want to freeze action, or you don't want noise in your image.
Ọfọn, ọ na-abụkarị maka ihe niile kpatara ihe ndị a n'elu - ịchọrọ ịchịkwa ịdị omimi gị n'ihi na ị na- agbapụ ala , ma ọ bụ na ịchọrọ ịkwaba ihe, ma ọ bụ ịchọghị mkpọtụ na mbido gị.