Images are treated as files. When uploaded to Drupal via an image field or wysiwyg button, a record is added to the file_managed table, and the image can be found under Content > Files.
Media are entities, which means they can have fields, view modes, and are reusable as referenced entities.
Reuse
A site may need to display the same image in multiple places. This is not possible with images by default, since both the image field and wysiwyg button will only upload a new image.