Ask HN: User-Agent – how do you use it?

Yesterday (Wed 19-Dec) here on HN we had a conversation about HTTP User-Agent headers in DNS-over-HTTPS requests. https://news.ycombinator.com/item?id=21819022#21821455 Some people made a strong case they are unnecessary for that.

(See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent for a description if you need one. )

Maybe they're becoming less necessary in web apps overall. So, the question:

How do you use them?

Do you use them in client-side Javascript? If so, what for?

Do you use them server-side? For deciding what content to send to clients?

Do you store them server-side? (If you store web server logs, the answer is probably "yes", of course.) How do you use those stored User-Agents?

If you didn't have User-Agent strings, would your stuff work?