You don’t have to use subcategories, and many large blogs (including WPBeginner) don’t. However, subcategories are helpful if you have a large category with a lot of posts that could be grouped into smaller sections.
For example, you might have a ‘Recipes’ category that contains a growing number of gluten-free recipes.
You can put these posts into their own subcategory so that it’s easy for readers to find them. You create a new child category for ‘Recipes’ called ‘Gluten-Free’ and move these posts into that category.