When first learning website development I was introduced to WordPress and was intrigued by the simplicity of creating highly functional websites that look great in a matter of hours. Now most of us here on Dev won't consider WordPress development as true "website Development." But I'm here to say that you can use WordPress in all kinds of new ways.
Headless WordPress is a way you can use the back-end database
of WordPress without the client end or front facing WordPress. Site editors will still use the WordPress editor and Dashboard to create pages, posts, etc.
connect via WordPress REST API for external outputs. Headless WordPress gives you the power of a CMS without all the additional scripts, files. This allows you to make lightweight, high performance pages.
Headless WordPress allows us developers to use newer technology such as React, Angular or Vue for the front end. The REST API will allow WordPress to send and receive JSON objects via a unique REST API URL.
The Down Side
One of the disadvantages of using headless WordPress is that plugins that do something on the front end such as Gravity Forms or WooCommerce won't work as usual. So you'll need another way to use these on the front-end of the site.
Headless WordPress is a viable option going forward to build future-proof your application with the ability to use newer tech. Also the benefits of the increase speed, quality and performance of the site make headless WordPress a definite win for more complex UI/UX.