The purpose of categorising posts is to organise your content. You group similar posts into sections, allowing readers to easily find all the posts on a certain topic. However, categories are more than just a list in the sidebar.
How Are Categories Ordered in a New Blog?
When a blog is new, naturally it will have fewer posts. Therefore, posts are grouped into rather broad categories. For instance; this post is currently categorised into the “Blog Usability,” category. Usability in itself is a huge topic, encompassing many sub-sections. As the blog is still new though, there is no need to break into these sub-sections.
Small blogs have a small number of categories.
As Your Blog Grows, Your Categories Grow
Over time, you post more and more, and the blog becomes larger and larger. However, because your blog is virtual, viewable only on a screen, you don’t see that.
Forget about the computer, and think about each post as a written document, and your blog as a filing cabinet. In the beginning, it may be suitable for you to use a separate drawer for each category, however, each page you add is filling up the cabinet. What do you do when there are so many pages that finding a certain one takes forever? You divide up the drawer. You take all of the pages in that drawer, and categories those, e.g. by name, by date or by topic.
Why should blogs be any different?