Blow ball is to heat up the ball, make it warmer and therefore lighter and better spin. Just try it yourself. Hold the ball and blow hot air on it, then place it between your earloop and the back of your head, the ball will stick in between.
Blow hand so that ping pong ball will not stick to it when you throw it up to serve.
Sweaty palm means a split second of delay as the ball is stuck onto the sweaty palm. Yes. it's all for practical purpose and there is physics behind it.