Since basketballs move at such low velocities, placing spin on the ball does not effect air resistance of the ball to any great extent. Rather, placing a spin on the ball increases the chances of the ball going into the net if it was to touch the rim of the net or the backboard as it often does. When a basketball has spin and hits one or the other, the contact made between the two objects will cause the ball to change its velocity in the direction opposite the spin direction as shown below.

Without spin the ball would bounce off the backboard at almost the same angle at which it hit the backboard because the balls elastic surface would help to conserve most of its energy and fiction would be a very small detriment to this conservation as shown below.


With spin, the ball loses a much greater amount of energy due to fiction for, as the surfaces come in contact, the spinning ball rubs up against the surface causing an opposing frictional force. The loss of energy that is caused from this action causes the ball to rebound with a much smaller velocity moving in a different direction and (with the help of gravity) a much lower angle creating a better chance for the shot to be successful.