The template-parts directory is not mandatory for FSE themes to function properly. WordPress will not throw any errors if this directory is missing from your theme structure. However, omitting this directory means you’ll lose access to one of the most powerful organizational features of Full Site Editing.
Template parts serve as reusable components that can be shared across multiple templates, such as headers, footers, sidebars, and navigation menus. Without the template-parts directory, you would need to duplicate code across different template files, making maintenance significantly more challenging and error-prone.
While technically optional, the template-parts directory is considered a best practice in FSE theme development. It promotes code reusability, easier maintenance, and better organization of your theme’s structure. Most professional FSE themes include this directory to take advantage of WordPress’s template part system and provide users with flexible, modular design components.