Why do some champagne and wine bottles...

This is called a punt and was originally the result of old-time glassblowing techniques. The blowpipe was attached to the neck of the bottle. After the glass was blown, the bottle was transferred to a tool called a punty. An indentation in the bottom was the result. Bottles were probably made this way to improve stability, since hand-manufacturing was subject to error. Today’s manufacturing techniques make it possible to create stable, flat-bottomed bottles, but punts are still used for finer wines, perhaps because people have come to expect them and also to make the bottles look bigger.