It's not that normal users are doing it in significant numbers. It's that other stakeholders are.
For example browser makers do that. Safari on iPads claims in the UA to be OS X on Intel, not iOS on ARM.
Another common use is for bots that are trying to hide they are bots. That might be malicious scrapers, non-malicious scrapers probing for whether changing the UA also changes the page content, or benign bots like software running phishing checks against links sent in chats or emails. It's not enough for these bots to just hide who they are, they want to look like a real browser.
I'm one of thosr who have, Microsoft office webapps love to misbehave with Linux useragents! I know Linux friends who spoof useragents for privacy reasons as well!
