Only purpose is to clean embedded crap out of the paint, by shearing it off.
You should do this first after you wash the car.Claybar carefully, using a good lubricant, I have successfully used Meguiars Quick Detail, and the claybars(ClayMagic for example), includes a lubricant, only do small areas at a time, then dry it and run your clean dry fingers over the spot, it should feel really smooth. Now go on to the next section.
A good mild cleaner can now help smooth out the swirls, holograms, scratches, etc., and this can be done by hand but its harder. I like to use a Porter Cable 7424 with Lake Country foam pads. The foam pads are different "grits" (for lack of a better word), and will usually get the marks out if they are not too deep. Along the way, the paint will now be crystal clear and you should not see any marks out in the sun. Good shop lights will help this process inside too. Now you need to put a paint sealer, wax, etc, on top of your hard work, to protect the paint. Some of these products give you great shine, gloss, etc., but are only short lived; others like say, Zaino, an acrylic polymer sealer, last really long and give you great shine and protection, including uv protection.
Know going into this that to do it properly, it takes hours - alot of hours.
But if you do the preperation really good, and keep up on it, it will last a long time, and eventually you will be spending less time out in the garage.
Its all into how much you really care about getting just an average finish or a super clean, smooth, deep finish.. Good luck with your project ! DanF