The formula wasn't why people cared at the time; it had been empirically known for centuries. What caused such a stir was he was the first person to prove that

The formula wasn't why people cared at the time; it had been empirically known for centuries. What caused such a stir was he was the first person to prove that for "most" right triangles there are no rational numbers P, Q such that PA = QC for leg A and hypotenuse C. That was what was earth-shaking.