The speed of a ball on the cushion of a table affects its reaction when it rebounds off a cushion.  For instance, in the diagram below, when you aim perfectly for a double at the tables natural speed, the speed which angle in equals angle out, then everything is in balance but lower the speed and the angle will widen slightly due to more contact time with the cushion and speed the ball up and the angle will become less wide due to cushion compression.  This can make or break your game from the start.  Some seasoned players still don’t use this to their advantage.