You could ask why not sell other things as well as books, or just other things that are digital.  You could.  But 1. you might be blunting your sword because its too general and turns people off because it doesn't stand out.  2.  it even more marketing, since these are all their own worlds, separate from each other.  With books there are more people then ever writing them while the markets to sell them in haven't really made any great leaps forward in favor of authors.  I would be networking and advertising in literary circles.  which are different from where other circles, like games or music.  I can present something targeted to one interest which all by itself is an enormous market.  later on more things could be introduced, with more people working on this and a name already.  my personal interest is here.  doesn't mean focusing somewhere else is better or worse.  just that i feel this particular market needs attention, and its where the big ideas are:  in books.  Being a literate society is important.  we've drifted into a more visual society. 
Music and games are nice.  But they don't change people's thinking.  or force people to think for themselves the way whole books can sometimes do. 

of the ideas i've had or heard, publishing or some kind of mastodon twitter thing (if it incorporates the token properly), seem the most promising.  But mastodon has never made any sense to me.  And i'm not sure anyone can compete with twitter.  maybe though.  I've laid out my own idea for publishing very. 

whatever direction we go it will be limited by our technical skills and capital.  the two hard realities of 21st century life.