A long standing tradition of the Indianapolis 500 is for the winner to drink from a bottle of milk immediately after the race. It is known as the victory lane bottle of milk which is a standing tradition that people now don't wish to see changed or broken. This tradition makes people feel good. It is in keeping with the fact that throughout the world, there are more than six billion consumers, who love to drink milk and milk products.

If you ask why milk is used, the answer is simple. Three time Indianapolis 500 winner Louis Meyer regularly drank buttermilk to refresh himself on a hot day and happened to drink some in Victory Lane as a matter of habit after winning the 1936 race. An executive with what was then the Milk Foundation was so elated when he saw the moment captured in a photograph in the sports section of his newspaper, he vowed to make sure it would be repeated in coming years. There was however a period between 1947-55 when milk was apparently no longer offered, but the practice was revived in 1956 and has been a tradition ever since.

More Info: en.wikipedia.org