Why most footers have a background color as a divider of section?

Why most footers have a background color as a division from the other content of the page? Wouldn't it be enough if they just used enough whitespace to separate? For example nielsen norman group: It has a grey background to separate the footer from the above content. But wouldn't the whitespace difference(using proximity principle) from the elements on the footer from the content above be enough to separate these two sections without using a different background color?