I don't know about anyone else (except maybe LornaJane), but when I blog, I generally do it to help future me.
Sometimes I blog about things that I couldn't find solutions to and had to work out myself (e.g. Node on CentOS 5.7), sometimes about things that I did find help with that I worry might disappear and sometimes even things that are easy to find, but I want a copy myself, just so I know exactly where to look next time I need it.
Sometimes it sucks, not having posts you can send out on twitter or submit to hacker news. I mean, how glamorous is recompiling coreutils? Then suddenly you get a comment from someone thanking you for your work. It helped them fix something they've been stuck on for ages. That's when it's really worth it.
So, blog for yourself. Blog about the boring stuff, about old outdated tech as well as the cutting edge. Someone will thank you for it, and that someone will probably be you in 6 months.