It gives a feeling of cohesiveness and consistency. Visitors find elements of a particular design on the home page which gives them familarity as they progress through the site. They don't always feel like they are going to a new page everytime they link within the website. If you use contrast to create a focal point be sure not to create chaos by using too many points of focus. It must be strong to be effective and to help bring organization to a page.
Backgrounds can be a real distraction to the layout and content of a page.