If they find any bugs or issues, I fix/address them before moving on to the next function implementation.
The significant advantage that this confers, is that the app is constantly in "release-ready" form; although incomplete.
Useful, for when we want to ask people to give us money. We just get an NDA, loop them into the TestFlight group, and let them have at it. No need for chaperones from Marketing, or sacrifices to the demo gods.