Why is displacement useful in finding the density of an irregularly shaped object?

in ancient times, people didn't have electronic or even effective weight scales. so to weigh things like gold or precious metals to sell was near impossible. the water displacement weighing method (created by Greek mathmetition Archemaidies)  allowed people to effectively get an approximated guess of how much the metals were worth... That's a mouthful! 

The kinetic energy of the liquid increases. hope this !