Let’s give a concrete and catastrophic example of something I’ve seen in the wild in a professional product. A developer there had obviously seen the application role[1] in the ARIA specs, thought “I’m building a web app”, and added it to their html element.
What role="application" means to assistive tech is: “I’m building a really complex application, so I’m going to handle absolutely everything for you, I don’t want you to have any default behaviour.” This meant that the web app in question was 100% unusable for any people using assistive technology, as that was broadly as far as they’d got with accessibility support.
[1] https://developer.mozilla.org/en-US/docs/Web/Accessibility/A...