When navigating a website it is quicker to hit the spacebar rather than scrolling down the page. This is also handy to keep your place. The spacebar sends you down a full screen length. This means everything on the screen is new information so long as you have not hit the bottom.
The spacebar will leap you down the screen. Holding down the shift key as you strike the spacebar will move you back up the screen. Chances are you will accidentally hit the spacebar key while reading a website and lose your place. Shift spacebar will jump you back to where you were.