Firstly, for customers; it works behind asset-blocking insane corporate firewalls, or for some poor schmuck in the sticks getting one bar of cell data and just enough to load our HTML, or simply to impress anyone running uBlock₀ in Advanced mode.

Secondly; because front-end Javascript bitrots faster than any code I've ever seen in four decades in tech. Reducing the quantity of JS in favour of letting the browser just do its job, is one of the best investments I can make against technical debt. JS can't be abandoned completely, but I measure every kilobyte jealously, and graceful degradation to baseline browser behaviour is my north star; simply using baseline browser behaviour is by definition my optimum limit for long-term productivity.

Thirdly; for folks depending on assistive browsing technologies. Yes, they do work in the face of dynamic web pages & SPAs, but many screen readers are still at their best when tabbing through plain HTML with a sprinkling of ARIA markup.

Finally, it's also a simple yet versatile mechanism. Good for anything from FAQ items to dropdown menus, even with default styles.