When the bolt is torqued, it elongates and stretches. The torque and resulting stress stretches the metal to the 'yield' point. Once metal is stressed to the yield point, it will not return to the previous stressed length. It will stretch further with less torque and stress, and is thus weaker.

It is a predictable way to apply a known amount of force to hold a head, but it means that the bolt is good for a 'one shot' use.

Best Regards,
Jim