As I've just started a research blog, I thought "Why do this?" was a sensible question to contemplate. (actually, I thought about it before I started, which seemed even more sensible...)
Sharing some thoughts...
Sometimes the best way to learn or come up with good new ideas is to talk to a fellow scientist. This is why chats over coffee and meeting up a conferences is so important (and why a few glasses of wine at a conference dinner can lead to valuable conversations). This works because of the ideas that are being shared.
There's no reason why talking should be the only medium through which to communicate in this way. And while a blog is less two-way (although please feel free to leave a comment!), it has the great advantage of being able to potentially reach a huge audience. It's difficult to have a conversation with more than a few people, and even a lecture/conference talk is unlikely to have an audience of more than a couple of hundred people (web-casts excepted). But there's nothing to stop thousands upon thousands of people reading a blog post!
Knowing your own mind
There's also a second benefit to writing one's ideas down in a blog post. It helps to clarify them. Writing something for an audience forces you to thing about what you're writing, to work it into a form that will be understandable by the reader, even to challenge your own assumptions. This can develop your own ideas and trains-of-thought in ways that simply thinking about them won't manage.
Returning the favour
Another very important reason is that I've found other people's research blogs very helpful in learning more about how to be a scientist (see myblogroll for some excellent examples). Not every blog will be useful for every reader, but it strikes me that if every researcher kept a blog where they posted their thoughts, observations and experiences, collectively that would form a great body of knowledge for other people to explore.
There are several great reasons why a scientific researcher should consider keeping a research blog. And there may well be others that I've not thought of yet, but that will occur as I post on more topics. Which is itself an illustration of why research blogging is a good idea :-)