The trouble is, keeping a website up to date can be a fiddle. At worst you have to delve into the raw HTML code that makes up the page, and change it all by hand. You might get better with practice, but it is still a deterrent. A blog is a fairly simple web page which has short, diary-like entries added in at the top. This is fast to produce, makes it clear when you have updated the page, and keeps the older material below so infrequent visitors can catch up.
Blogs are now hugely popular, for all sorts of subjects.
Some review other websites, while others exist as a centrepoint to an online community of interest. Others are used to keep colleagues in touch with each other, or manage projects. There are also a huge number that contain vaguely confessional musings - a new web style which for many has come to define what a blog is. Blogs are, in short, hugely versatile. And, to make things even easier, there is a web service that makes setting up your own blog a 10-minute process.