The main purpose of the categories is to organize your pages. It can be very usefull to order specific pages in specific categories or subcategories.
Also you can use categories to use another layout file, add a segment to the URL, or have an automatic navigation built upon a category.
When adding a category, 3 types of information should be provided.
Name: This is only for your own reference, but should match the name of layout file in your project.
URL: The URL for this category, all pages within this category start with this URL, and then add their own slug.
A category can also have a preset of sections, so each page within this category will have this predefined set of sections on creation.
To do this, you edit a category and go to the preset tab.
That's where you can add sections to a category. When added, all new pages within that category will have this preset of sections already present.