The Stockade is the Alliance answer to the Ragefire Chasm instance, and is the only dungeon available in an Alliance City.
The heavily-guarded prison is experiencing a riot inside, and only you and your other party members can stop it, since the guards there are extremely incompetent. They can't get five feet into the dungeon without needing a place to rest, and a priest to heal them.
Prior to Cataclysm, the riot was started by Edwin Van Cleef's lieutenant, Bazil Threed, who got himself locked up in order to take down Stormwind from the inside. He was the final boss for the a number of years.
When the world changed, and adventurers had killed him already, a new boss needed to be added, and so the game designers decided to make fan favorite Hogger the final boss in the dungeon.
For most Alliance players, Hogger, the leader of a trogg gang in Elwynn Forest was the first "Elite" mob that they encountered, and most were thrown off guard by how tough he was if attempted without a group and slaughtered. Players who do the Elwynn Forest questline are actually responsible for getting him arrested in the first place, so this is a neat way to wrap up the storyline.