Because different objects heat up in the sun in different ways. For example, a sheet of black paper heats up more than a sheet of white, even if they lie side by side. If we want to measure the temperature to which the sun can warm the thermometer itself, then we should put it in the sun, and if the air temperature, then it is logical to place it in the shade.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.