The most important thing about a blog is obviosly the landing page. Not the landing page as in some page that annoys you and tells you to click next but the home page. One of the reasons this is so important and especially for search engines is because what is on your frontpage depends what traffic you will get from search engines.
Most blogs have the category with the post and the categories that fill the front page will dominate your searches, although the category itself will not show up in search engines the tags in those posts and the posts on the frontpage their categories will rate higher than those that do not appear for the moment.
As soon as you blog about something else in a different category that certain topic will appear higher in search engines as things that are not on your frontpage and directly link to the categories.
That is why its important to have and blog about all your categories as often as possible.
