Scott Adams, creator of Dilbert, explains it well.
When I first started blogging, my future wife often asked about what my goal was. The blogging seemed to double my workload while promising a 5% higher income that didn't make any real difference in my life. It seemed a silly use of time. I tried explaining that blogging was a system, not a goal. But I never did a good job of it. I'll try again here.
Writing is a skill that requires practice. So the first part of my system involves practicing on a regular basis. I didn't know what I was practicing for, exactly, and that's what makes it a system and not a goal. I was moving from a place with low odds (being an out-of-practice writer) to a place of good odds (a well-practiced writer with higher visibility).
The second part of my blogging system is a sort of R&D for writing. I write on a variety of topics and see which ones get the best response. I also write in different "voices". I have my humorously self-deprecating voice, my angry voice, my thoughtful voice, my analytical voice, my half-crazy voice, my offensive voice, and so on. You readers do a good job of telling me what works and what doesn't.
When the Wall Street Journal took notice of my blog posts, they asked me to write some guest features. Thanks to all of my writing practice here, and my knowledge of which topics got the best response, the guest articles were highly popular. Those articles weren't big money-makers either, but it all fit within my system of public practice.
My writing for the Wall Street Journal, along with my public practice on this blog, attracted the attention of book publishers, and that attention turned into a book deal. And the book deal generated speaking requests that are embarrassingly lucrative. So the payday for blogging eventually arrived, but I didn't know in advance what path it would take. My blogging has kicked up dozens of business opportunities over the past years, so it could have taken any direction.