Perhaps she is curious. Perhaps she has some masochistic feelings that are vented through the fantasy. Pornography breaks cultural and political stereotypes, so that each woman can interpret sex for herself. Anti-feminists tell women to be ashamed of their appetites and urges. Pornography tells them to accept and enjoy them.
Pornography can be good therapy.
Perhaps they are away from home, recently widowed, isolated because of infirmity. Perhaps they simply choose to be alone. Couples also use pornography to enhance their relationship. Sometimes they do so on their own, watching videos and exploring their reactions together. Sometimes, the couples go to a sex therapist who advises them to use pornography as a way of opening up communication on sex.