Beta testing has never been this simple.
Both developers and testers are aware of the headache known as ad hoc distribution. Packaging a build and profile, emailing testers, dragging everything into iTunes and sync until it starts working. With TestFlight, developers simply upload a build, and the testers can install it directly from their device; no iTunes required, no manual syncing. All this on iOS4, without jailbreaking.
TestFlight also tells developers who has tested and how they have tested builds. It lets developers invite testers via email, they can forget about gathering and managing UDIDs.