For usage, often used for accessibility testing.
With WCAG becoming more and more relevant (and legally required depending on location), testing for accessibility is becoming a large concern.
Screen readers and a lot of assistive technology will read and interpret the site according to the markup, not the styling.
Standard best practice is to look at the site with zero styling and ensure it still makes logical sense.
The menu option provides a quick and easy way to do that.
With WCAG becoming more and more relevant (and legally required depending on location), testing for accessibility is becoming a large concern.
Screen readers and a lot of assistive technology will read and interpret the site according to the markup, not the styling.
Standard best practice is to look at the site with zero styling and ensure it still makes logical sense.
The menu option provides a quick and easy way to do that.
That's one common use anyway.