Why bowler rub the ball to his/her trouser?

As when the innings started with a new ball the ball gone straight to the batsman with less swing or little reverse swing which provides an advantage to the batsman as he/she hits the ball easily through the middle of the bat whereas this called up as an limitation for the bowler as he/she finds difficulty in make the batsman bowled or stumped by the keeper.

When the bowler rub the ball to his/her trouser the ball keep shine and it make little slippy so that it can easily slip over the edge of the bat and bowler will able to achieve wicket an another alternative is to use saliva.

However you rub the ball the empire of third empire will not say you anything but if you violet the rules of ICC you have to pay the loss.

Thank you