At first it was all about presenting data in an original looking way. In the end it was about pushing political ideas in your throat using a plain bar graph. It was not about sharing something interesting you found but about taking advantage of a captive audience.
Most of the time the point is to build up the account’s karma and history. Then it is sold to a company, politician or whatever. In turn they use it to astroturf reddit. Which is much more effective when the account has enough karma and is old enough to post in various subreddits, and stands up to scrutiny thanks to months or years of seemingly normal reddit usage.