Why Do Treebutts Exist?

Not to steal the chicken heads' (or lack thereof) thunder, but this came up while trying to search "Why do treebutts exist?" and in the process of typing "Why..." Google suggests some pretty questionable phrases such as "Why can't I own a Canadian?" (Thanks Lindsay).

I probably should've given a forewarning--examining this picture proves to be a little nauseating. But hell, we're ahl adults here.