It just helps to show they live in the 'real' world. They're surrounded with familiar brands and products rather than fictitious ones. They hand out in McDonalds, they shop at gap just like everyone else. Well I say everyone else, not me, since my town had neither a McDonalds or a Gap, or a shopping mall, but you get the idea. It also helps to promote the idea that that it could be any town (even your town, as they used to say).
Either that or it's a commentary on globalisation.
