Feature flags are a great way to selectively introduce new features. it allows you to experiment and commit incrementally.
The only down-side to feature flags all the extra code, and in particular going back later together rid of all the crusty flag code flagging you feature on or off. This kind of tech debt can really pile up over time.
Apparently Uber uses feature flags in the thousands and without remorse. So they came up with this automatic way to wipe out your stale, disabled code. Perfect name, too!
Via iOS Dev Weekly. See also: The Mother of All Feature Flagging Systems for iOS