I think rjw got close to it. It's a quality control thing. The Internet greatly lowers the barrier to shoving information in peoples' faces. It used to be - if you were a musician, you had to get a label to financially back you. Filmmaker? You had to get a studio/distributor to do so. Writer? Get published in NYT or get a publisher to pick you up. The financial barrier imposed a means of quality control.
Nowadays, anyone - no matter how crap they are - can get a link to their work or ideas in front of your eyes. Given our limited attention span, an easy way of filtering out this noise is the notion of whether content is "vouched" for or not by others. Self-posted vs posted by someone else is an quick and easy cut-off for this.
This is an issue that some sites attack in an indirect/inadvertant way (e.g. Reddit/Twitter/Soundcloud/Pandora etc.), but AFAIK no one has yet to attack (in a rigorous manner) the issue of quality control. It's a challenging problem in part, because it's so subjective.