it does the following things (remember that each club has certain carry # and run #, for example: for 220 3w it might be 200 carry and 20 run)
spin does the following: top spin decrases carry increases run, bs the oposite.
in certain winds or holes where pin is close to the edge, bs is useful cause it carries more.runs less = stops faster
same for if you have head wind and want few more yards outta 3w, u'd use top spin , fly it lower and run it longer
all of this requires practice and # crunching:) = ie fun parts of the game:)