Creating categories helps your users filter and easily access articles and post their most interested in which provides and enhanced user experience and who doesn't want that? 
Step 1. From your WordPress dashboard select "Posts" and open the blog post you'd like add a new a category for
Step 2. Scroll down the sidebar until the Categories heading appears
Step 3. Select the "+Add New Category link"

Step 4. Type the new category in the box 
Step 5. Select "Add New Category" 

Step 6. Select the new category check box
Step 7. Select update and your blog post will now be categorized