On all social media platforms, there are two types of people, and people who are a mix of the two.
One: People who believe what they have to say deserves an audience.
Two: People who find value in what other users produce.
It really is as simple as that.
Bloggers, and people with Twitter, Facebook and any other social media account fall into one of those two groups. People who say they write/post for themselves, but put it on the internet, are lying. If they posted purely for themselves they would write it in a journal. The act of putting something on the universe's most public medium, the internet, denotes some intrinsic hope of a person other than yourself, consuming your content.
Even those of us who complain constantly about the mundaness of the majority of people's content still continue to log on, and read it. If zero value was found in consuming other's content, we wouldn't look at it at all. Value is defined as a fair return or equivalent in goods, services, or money for something exchanged. Regardless of complaint, we exchange time out of our life for the perusal of what others place on their social media stage. Even if we complain about it's uselessness, we exchange our time for it.
We are in an era where it is relatively easy for someone who believes they have something of importance to say (whether they do or not) to have a platform to say it, and an audience to consume it.