The elephant in the room: it is used for porn. Porn drives many things IT. As it is still forbidden or restricted in many countries, most user cannot just point their browser to porn sites. So they download or copy pics and vids to SD cards. Porn on the phone is so convenient for students, young workers (because they do not hav e their own room with a good PC screen to "relax" alone).

Why do you think most (and best) Android file explorers are Chinese?