Most questions, including the ones you just asked are answered there.
However, to help answer your questions, the purpose of A-spec & B-spec points is equivalent to a "score" in other games. They simply tell you how many points you have acquired. They are just one of many challenges GT4 offers players, but they are not required, nor do they effect game play by changing performance, unlocking features, or adding to your game completion percentage.
B-spec skill points (Machine, Course, & Battle), do effect game play by improving your B-spec driver’s skill levels.