By speech enabling your blog, you offer readers the option of listening to your blog content. This can be incredibly useful for people with bad eyesight or for people doing a lot of multi-tasking that just prefer to listen to the content being read as they are doing other things.
But honestly, I think the best reason to do it, besides the fact that it is free and easy to do, is just that it is way cool. It can help to differentiate your web blog from all the millions of other web blogs out there.
So how do you do it? The easiest way I have found is to use a new product called webReader. webReader offers various subscription services that can be used on commercial web sites, but they also offer a free version for personal websites and blogs. The only downside is that the free service includes an advertisement that is read at the beginning of every post. But the ad is very short and I think the benefits of having the service far outweigh this one minor annoyance.