Why do bowlers or fielders rub the ball on trousers?

Why do bowlers or fielders rub the ball on trousers?

As the ball is used for an extended period of play, its surface wears down and becomes rough. The bowlers will polish it whenever they can usually by rubbing it on their trousers, producing the characteristic red stain that can often be seen there. However, they will usually only polish one side of the ball, in order to create ‘swing’ as it travels through the air. They may apply natural substances (i.e. saliva or sweat) to the ball as they polish it, but any other material is illegal. Since the condition of the cricket ball is crucial to the amount of movement through the air a bowler can produce, the laws governing what players may and may not do, to the ball are specific, and rigorously enforced. It is illegal for a player to rub any substance apart from saliva or swear onto the ball, rub the ball on the ground, scuff the ball with any rough object including the finger-nails, or pick at or lift the seam of the ball