Why did the ancient egyptians use X-rays?

Answer 1

Short answer, they didn't.

X-rays are usually formed by strike a metal surface with high energy electrons, these are generated by using very very high voltages and the ancient Egyptians did understand any of the principals that would needed to generate or use X-rays.

It is important to keep in mind, that while the ancient Egyptians accomplished a lot, they weren't some kind of super advanced race visited by aliens as some authors would have you think. They are usually out to sell books and are often times disingenuous.