Category is a completely optional thing you can have in your blog post.

In practice, I always assign my blog posts a category, even if I do not want to use categories. That's why I seed the categories database table with a "main" category.

So if I change my mind later, I already have a category assigned to my existing blog posts.

You can have just one category in your blog post.

The page that lists blog posts for a specific category can be customized with an image and specific content: