You badge in to give the illusion of security and so they can track that you are in the building, in part for their own liability in case of an emergency ("Yes

You badge in to give the illusion of security and so they can track that you are in the building, in part for their own liability in case of an emergency ("Yes we accounted for Jim's whereabouts during the fire, he entered the building at 9:05."). You don't need to badge out because the magnetic strip likely automatically tells them when you left the building by virtue of walking through the door.