If you mean while the point is being played then they shouldn't be as that is illegal, but if you mean in between points when they touch the sides or edges of the table, that is because they are getting any sweat/moisture off their hands so they can grip the bat properly
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Its for encouragement and to try and stay positive. You only have that one person to play with. They can either motivate you to do your best and try and get the next point, or throw their racket to the ground and give up. I'd choose motivation :)
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