What is the purpose of bleaching trees?

Trees are whitewashed in December/January to prevent damage to the bark caused by temperature differences between day and night. If it is warm and sunny during the day, the trees wake up from winter dormancy and start growing, after which they can freeze at night. The white color reflects the sunlight during the day, so the trunk does not heat up.

Trees are painted with slaked lime (lime mixed with water) to protect them from being chewed and from pests.

From what I know, they are whitened with lime. And there are two reasons: 1. Insecticidal - these pests that enter through the trunk are deterred. But how effective is it? 2. But probably the more important reason is the second reason - the bright color reflects the spring sun rays and slows down the resumption of sap circulation in the trunk. This is probably to prevent the trunk from cracking in case of late strong frosts. So whitening for this reason makes sense mainly in late winter - for example in February.

This is some kind of insecticide, I guess, to prevent ants from crawling up the trunk to the fruits.

Whitewashing trees aims to protect the bark from the harmful effects of sunlight and chemical pesticides. This method is used to increase the resistance of trees to pests and to inhibit the growth of fungi. Special preparations containing substances that protect the tree bark from adverse weather conditions are used for whitewashing trees.

Protection against rats, mice, deer, etc.

My grandmother used to say that it's against parasites and fungi, and houses should be painted light blue so that insects associate it with water and don't fly towards it.

This is supposed to protect the bark from cracking due to temperature changes.