The category defines the focus of your blog. Therefore it also determines they type of target audience for the blog. For instance that your blog is animal related, then the wordpress themes should also be associated with the content. If you are aiming for people concerned with cooking then the same thing applies.
Searching your category will assist in getting the correct theme, whether it is right sidebar themes or left sidebar themes. This will save you more time and effort in the course for establishing the theme and doing some editing. Themes are usually categorized from travel, music or business.