hen creating a WordPress theme, there are a number of things that you need to consider such as images, links, widgets, post formats, galleries, lists, comments, page navigation, etc.
You need to make sure that your theme is ready for all these elements that a typical WordPress site is supposed to handle. Theme unit test data helps you test your themes for HTML tags, formatting, navigation menus, comments, widgets, etc. This allows you to create a theme that is truly compatible with WordPress. It is extremely important to test your theme with theme unit test data when you are planning to publicly release a theme.