It’s an alternate app store primarily for FOSS software, including a lot of stuff that you can’t find on the Play store. It’s very well vetted and there’s zero reason not to use it.

If you want a more specific example of something worth getting on F-Droid, there are several adblockers which work significantly better than anything you’ll find on the Play Store, simply because the Play Store largely prohibits apps which directly affect the operation of other apps.