It is a safety precaution.
It is well known that exposure to very high levels of RF (Radio Frequency, microwaves included) radiations can be harmful to biological tissue due to their ability to heat the tissue rapidly. (Same principle is applied in working of microwave ovens used to cook food.)
Tissue damage could occur in humans due to our body's inability to cope with excessive heat that could be generated due to RF exposure.
The eyes and the testes, are particularly vulnerable due to relative lack of available blood flow in these organs to dissipate the excess heat thus produced.
Scientific literature is full of reports describing the observation of a range of biological effects resulting from exposure to low levels of RF energy. As much of the research is not carried out on whole bodies it has not been concluded definitely that such effects could constitute human health hazards.
Studies have also shown that environmental levels of RF energy routinely encountered by the general public are typically far below levels necessary to produce significant heating and increased body temperature.
However, in situations, particularly in high-powered RF transmitters, the recommended limits could be exceeded for safe exposure of human beings to RF energy.
In such instances, safety and restrictive measures are necessary to ensure the safe operation of these installations by engineers.