That seems to be the question for some scientists.
For those of you wondering why anyone would spend time blogging or reading blogs, there are a few discussions out there by scientists who've started blogs. The links below offer some well-reasoned and well-written commentary on the topic.
One of the most compelling reasons is that blogging helps sharpen writing skills, particularly with respect to developing well-reasoned arguments or explaining your science to non-scientists. I find that students and many early-career scientists have great difficulty explaining their work in everyday language.
This blog entry explains how to start a blog and some pros and cons: go here
For those of you in tenure-track positions, here is an interesting blog entry by someone (female) in the process (her tenure package included her blogging activity as a positive example of scholarly activity): go here
And here is the result of her tenure review.