Shining the ball is particularly essential in Test cricket, where bowlers need to generate sideways movement with the old ball in order to stay in the game. Bowlers generally shine one side of the red cherry while keeping the other side dry in order to obtain reverse swing.

from Times of India https://timesofindia.indiatimes.com/sports/cricket/news/sweating-over-new-rules-iccs-proposed-ball-tampering-law-divides-pacers/articleshow/75385325.cms