Case nÂș1: Sticky footer
There is a very well known css trick to stick the footer to the bottom of a page if you know the height of the footer: Ryan Fait's Sticky Footer.
But in modern day responsive world you never know the height of your footer. Even if you didn't do responsive layouts, if you use a CMS the odds are that the footer content is editable by the site owner, so you'll never be certain of the height.