This function will allow you to check if the current post or archive page is assigned a specific category. I use it often to apply category-based body classes and special styles for category archives.
The WordPress environment gives a lot of power to developers. You're free to open up and study, alter, or use any of the code that is compatible with the platform.
But the best part about WordPress development its powerful set of native functions (conditional tags, taxonomies, hooks, etc) which give the developer great control over how and where to display content stored in the database.