The dimple that is at the bottom of many wine bottles is called the punt. There are a lot of theories about why wine bottles were originally made with a punt, but my guess is that the only reason they are still being used today is because so many segments of the wine industry are hellbent on maintaining tradition. (That's true even when it is to the industry's detriment, but don't get me going on that.)
You would think that wine bottles made with punts would be made of colored glass, another tradition. Not these wine bottles. They're made of clear glass.