Yes, provides the paint with shine and protection, but paint doesn't rust. The paint and primer stops the metal from oxidising. As above, spray lacquers are too porous to go on raw, untreated metal. Water molecules will get through, then the lines start spreading, then the lacquer just peels off.
Yeah you have a base coat (the colour) then a clear coat (the laquer) the clear coat is what gives the paint it's shine and protection.