Question: What is the purpose of whitewash I see on trees’ trunks?

Answer: That is an old-fashioned method of inhibiting insect invasion. They were repulsed by the reflective white surface. It was used especially for protection against borers such as the damaging peach tree borer. It gave way when better borer-preventive insecticides became available. Many of those are now gone from the market, and borer control is more difficult than it has been for many years.