Alignment exists to help teach people to play a character in a ROLE PLAYING game. In these games you take a role and act out a persona that is not yourself. It was added because without it, players tend to play themselves and metagame as they themselves think rather than addressing situations that arise as their character. Alignment is a tool to stop people from going OOC (out of character). It is an easy-to-recognize box the GM can reference to see if players are in character and reward them or penalize them accordingly. Old DMs like Gygax did encourage penalizing players who kept going OOC, especially if you are getting your powers or tools from some sort of in-game patron to serve their cause. Using cleric powers inappropriately or using a magic weapon gifted to you by a powerful lord against his protected vassals would have serious consequences. Having an alignment to follow helps players consider how their persona is inclined to act in a given situation.
