In no particular order, here are some reasons:
People said I should.
Like Augustine, writing clarifies my thinking.
Even though I have written three books and several articles, I have loads of material which is better suited for the provisional nature of blogging.
Related to the previous reason is that blogging is a great place to test out your ideas.
You get multiple perspectives which help sharpen your convictions while also exposing errors in your judgment.
I gave up Twitter a long time ago due to my frustration over receiving spam, best recommendations on pizza restaurants, and a daily deluge of silly sound bites.
My gratitude goes out to Blake Freeburg, Terry Cokenour, and Kent Major who made this blog a reality.