Prevention of noise and electric shock!
Insulation have various purpose such as stability of voltage, prevention of noise and electric shock because Electrical Appliances and Materials Safety Act establish insulation of corresponding product in electrical and electronic industry.
Insulating measures when you use metal screws is insulation coating needed. But the cost is increase. On the other hand Plastic screws are optimum products for fix of transistor using in heat sink, printed board, heater parts, antenna parts.
* Tolerances of material are not guaranteed using of plastic screws.