It's all about optimization

3.0.2 Don't use HTML wrappers as a box model hack

Difficulty with the box model in quirky browsers prompted web layout artists to employ wrapping techniques to get their layouts padded correctly. Using a Strict DOCTYPE, we no longer have to wrap our elements.

Example One

Using a quirky browser or a Transitional DOCTYPE, to get browser consistency you would have had to do this:

BEFORE

Using a STRICT doctype, you no longer have to put wrappers around your box:

AFTER
Negabytes gained in this example: 37