Isn't the point of an UI to have an ... UI after all? The only ones that I can see being headless here are customers buying in to headless UIs, headless CMSs quackery.
Try building a fully accessible dropdown from scratch in React? This is what these libraries solve.
An example from Radix UI: https://youtu.be/pcMYcjtWwVI
Headless CMSes are useful as well once you exit the realm of blogs and WYSIWYG website builders.