This skill is only used in extended dialogues. Besides being used to present a convincing argument, and getting the NPC to agree with you, it is used to present a convincing lie and getting the NPC to agree with you. The difference between an argument and a lie is that if you blow the argument, the NPC generally will not want to slit your throat. You never get to use this skill actively while talking, as the lines that Speech rolls are used on are not marked. A higher Speech skill is useful for getting your way. You can use the Speech skill actively to start a conversation, but just clicking on the person is usually easier to do.