There are many files in your WordPress directory. These files can be downloaded back from wordpress.org site. Some of them can be retrieved back but some can’t.

The wp-content directory contains all your installed themes and plugins including all your media files like images, audio or video files which you uploaded on your site. Hence, it makes it personal and unique.

The wp-config.php file consists of your Database and other personnel options which make it also a unique file.

Both the above files can’t be replaced by other default files. Hence, they need to be backed up.