A cheat for the author
While testing, Hashimoto could not finish Gradius, so he added a code that gave his ship every power-up. He forgot to remove it, and players discovered it. Konami then deliberately put the code into other games.
Contra and 30 lives
In the American version of Contra, the code gave you 30 lives instead of three. For many kids it was the only way to beat the game, and the code became part of pop culture.
Easter eggs on the web
Websites, apps and games still hide the code as a secret joke. It does something on this website as well. Just type it on your keyboard anywhere on the site.