Just because I like to shut people for life: Advertisement and decoration, you don't need an HTML element with title "pen for background decoration" where you want people to focus in your product. The other one is separation of concerns, many times the programmer and the designer work together, but is sometimes unhealthy to give the designer access to the templates (or php, whatever), is way less dangerous to give him access only to CSS, so using stuff like this helps in such case.