Generally 2 types, prostitutes or a form of sexual perversion defined as: "to cause to turn aside or away from what is generally done or accepted".
Online prostitution including but not limited to: pornographic video or photos, text, fetishist video or photos, courting, or dialogue. Typically done for ad revenue, attracting donors, publicity, etc.
Sexual Perversion, from what I've seen a lot people do it because of sexual abuse, coercion from SOs, peer pressure, outlet for fustration, the material was taken/uploaded without knowledge or consent, or they're fabricated images.
Childhood abuse and to a lesser extent, sex trafficking play a large role in this. Not to mention the hyper-sexualization of children and teenagers, who are bombarded with 1000s of ads and other means of propaganda on a daily basis.