How does setting the height of elemets html and body to 100% help me in setting the height of an element to the device's screen height? : [I tumbled upon this question here][1]


  [1]: http://stackoverflow.com/questions/12172177/set-div-height-equal-to-screen-size

Look at the answer, 
He put the height of the body and html to 100% 
which helped him solve his answer,

Can I get correct explanation for this?
why that happens? what if I don't use the overflows? 
or don't put 100% height to the html or body element? 