If you cool air at constant moisture content, at some point relative humidity reaches 100% and water droplets form; that temperature is the dew point. Air at 25 °C and 60% humidity has a dew point of about 16.7 °C: every surface colder than 16 °C that this air touches will sweat. This is exactly why the outer face of a cold room door, panel joints and pipe insulation sweat or ice up.
Because the evaporator surface runs below the dew point of the room air, moisture condenses on it; if the temperature is below 0 °C this moisture turns to ice and defrost is needed. The greater the gap between surface temperature and dew point, the more moisture is pulled out of the room. This is an unwanted effect in vegetable rooms that must stay humid and a wanted one in drying rooms.
To prevent condensation, the insulation is chosen thick enough to keep the surface temperature above the outside dew point; in very humid climates (coastal cities) thicker panels and good door gaskets are needed. The heater wires in the door frame also keep the frame above the dew point, preventing freezing and the door sticking.