Maybe not the only reason, but this was definitely one of the main reasons I used Authy. Over time, the product has been getting progressively worse... When I first started using it there was a Chrome App you could install which was great because it could work on "corporate" machines where I wasn't able to install the desktop app. That went away a long time ago, but at least we had the desktop app on Windows, Mac, Linux. Although, at some point Authy was only available on Linux if using Snap, which ruled it out for me (although there is an unofficial Flatpak now). So now they are getting rid of all desktop apps which will be the end of my Authy journey and this will also be the last Twilio product I use, since I've had recent bad experiences with some of their other products.

I feel stupid because I’ve been recommending people use Authy for the past year “because it has a desktop and mobile app.”

Yeah its super convenient. I do keep an old backup phone around just in case and occasionally turn it on to sync, but also have the desktop app.

I've only ever used it on mobile, so no. For me the reason was sharing TOTP between phones in case my primary gets lost or damaged.

That's fair. I should say, it was my main reason for using Authy as there wasn't anything else out there that could do synced mobile and desktop easily for free.