The purpose of Medium is very simple. It is a content writing site provided by Ev Williams to try to find another way to reimburse writers outside of advertising.

It is not a blogging site. A blogging site is a site where people read about the personal lives of others, and where one has to get one's own followers.

A content writing site is a site that pays money for good writing (generally through advertising), and where good writing is determined by the traffic it gets. This means one has to understand SEO.