Could you and would you are actually conditional phrases when used in context e.g. would you mind helping me out with something? Could you please write your name down for me? These are polite ways of asking questions. 'Could you' can also be a past tense (imperfect) if used in the following way: When you were young, could you (meaning were you able to) go out with your friends? Does that make sense?

You are correct that they are past forms of 'can' and 'will' and past forms are usually used to indicate a distance in time but they can also be used to indicate a distance in status/social standing, especially in the context you are using. So you use these polite forms because you are 'more distant ' from the person you are speaking to as opposed to a friend. Hope this helps Bob