What did the extended stay aboard Starliner provide Boeing and NASA concerning the hardware?

Invaluable real-time data on the leaky thrusters under steady thermal and power loads

Although the delay was costly and created scheduling problems, the extended duration spent attached to the ISS offered significant, unplanned benefits for the testing and certification process. For Boeing and NASA, having Wilmore and Williams aboard turned the wait into an unplanned, extended checkout of the hardware. Every additional day allowed mission controllers to monitor the specific behavior of the affected RCS thrusters while they were under steady thermal and power loads in space. This flight-specific data, unobtainable through ground simulations, proved highly valuable for validating the vehicle's behavior and building the necessary confidence for future operational missions.