Why is the universal law of gravitation one of the greatest scientific discovery in the history of science?

Newton discovered the law of gravity which led to understanding how the planets move. He also discovered the laws of motion which led to improved machines. And calculus which also helped maths.

Everybody already knew about gravity before it was written down as a fact for as I know if it wasn't for gravity, I'll never get that dang water to stay in my glass....

The discovery did not change how the world works but rather it changed the way we understood how the world works. I refer you to Quentin's answer for examples of this increased understanding.