The purpose of a punt (the indentation in the bottom of most wine bottles) is to give you a more stable grasp of the bottle while you pour.
While it’s tempting to pour a wine by holding the bottle with your thumb inserted in the punt, the indentation in the bottom of bottles was never intended for that purpose. Originally, punts were a way of preventing the jagged pontil mark—the point left over after a glass bottle was blown and shaped—from scratching the surface of a table. By pushing the pontil up into the interior of the bottle, a punt was formed and the table was saved. When mold-made wine bottles were introduced, the punt remained, since it adds stability to the bottle when it’s standing upright. With Champagne bottles, however, the punt has even greater purpose. During the second fermentation, which ultimately gives Champagne its bubbles, six atmospheres of pressure is built up inside the glass wall of the bottle. The Champagne bottle’s prominent punt allows for a more even distribution of pressure inside the bottle, preventing the disastrous explosions that were a common and serious problem for early Champagne makers.