Please refer to the following weblog article which I wrote specifically to address your question.
Berger Bulletin » A Better Ballistic Coefficient
As a real short summary, the main advantage of a G7 BC is that it doesn't change as the bullet slows down like a G1 BC does. This makes it possible to calculate long range trajectories that are more accurate at all points.
Feel free to ask any further questions after reading the above article.