And that he included projects that were wildly successful (YC) as well as projects that didn't get the traction he wanted (Bel). To me it shows two things: 1) that some of the most successful people make miscalculations about what the world needs. 2) that a few of them, like PG, are courageous enough to admit that.
Personally, reading PG's essays and then going through YC years later changed the course of my life. That is, the essays are one way to change the world for the better, and they are one he's very good at. I'm glad he's writing them again.
For any one new to tech or startups, they will introduce you to new ways of thinking, and save you from a lot of mistakes.