Categories impose structure on your posts. Say you write about a couple of different topics. You could just write them all under a default category and leave them all jumbled together. As a designer, that could mean posts about case studies mixed in with design tips mixed in with day-in-the-life stuff. One of the ways we make it easier for the people reading our sites is by grouping alike posts together. Categories allow you to do this. You can split our examples into three separate categories of: Case Studies, Design Tips, and Day-in-the-Life. You can also have sub-categories to split things down further when categories become larger. So you could have Design Tips, then underneath Branding, Coding, Quick Tips. Posts can belong to more than one category where it makes sense, whatever makes them easiest to find for your readers.