Developers have to test their software before releasing. Because different iOS devices exist, and different configurations can affect things differently, it's especially important to test out on various devices if at all possible.
That's where TestFlight comes in. Rather than trying to get your work-in-progress App on the App Store and hope to test that way, why not distribute it directly so you can get the feedback you need?
It's simple as the image below shows:
1) Drag the IPA into the designated space.
2) Shoot an email to your potential testers.
3) See how many of them install it.
See? Easy as that.