> Why on earth would the backend function even take an email?

In principle, it could be designed to do so to handle cases where a new email address has been confirmed out of band, e.g. for an account representing a company or a political office. But that's a relatively unusual situation, not something you'd want to be available to every user writing in. (Even if you had an all-human support department, this sort of functionality would only be available to a select few agents.)