Good question. I could spend several months positing answers and maybe I will.
Answer number one: because everybody else is.
Not good enough. If everybody else stuck their hand in the mangle, would I? (shades of my grandmother there, and if you don't know what a mangle is, go and look it up).
Answer number two: because people I like and admire do.
Somewhat better. But then, people I like and admire are enduring power cuts and curfews in Nepal, planting trees in Edinburgh, and dicing with death in Gaza. I'm not.
Answer number three: because I think I might be good at it.
Fair enough. Fitness for purpose is a place to start. Of course then there are all the other questions like what am I going to blog about, and who is going to read it when I do ... but let's just suck it and see, shall we?