I can guess why - but I have no experience - I just guess.
Normal Domestic Hot Water is heated to at least 60°C to kill any bugs in it. If you were doing a Hot wash your machine would open the hot water fill valve and bob would be your uncle....
But if you were doing a 30° wash the cold fill valve would be opened and the water heated to temperature.
Consider what might happen if the Hot Fill Valve developed a fault - you would be unaware of the fault whenever doing a cold wash but on a Hot wash cycle the pressure switch that shows sufficient water in the machine would never be satisfied - the washing cycle would not commence.
How does that sound?
Just guessing...
August 2015