Sweat (or humidity) in the glass drastically reduces the grip - this results in the ball interacting with the glass in an unexpected way: both at a higher speed (since no energy is lost gripping), and at a different angle (due to lack of grip). So, if you're defending and you let the ball hit the glass, it seems to quickly fall towards the ground. And if you try to hit a ball against the back glass, it will fly high and away, most likely against the opponents walls or out of the court. So overall makes it much more unpredictable and harder to defend.
