Why do people overcoded webpages? [on hold]

I'm in web programing for more than 1 year now and I really like it. I have created many webpages for my friends and myself, but there is a difference between my pages and the one I see on the internet. When i see there code they have a lot of html, css(that overwrite it self), lot of elements with class attribute. I mean, I can get the same result with simple code.(And I'm not talking about big pages like facebook and others). Why is this? THANKS