The reason that you want to use
CoolingMachines cm = new Refrigerator();
is that you can later easily use a different CoolingMachines. You only need to change that one line of code and the rest of the code will still work (as it will only use methods of CoolingMachines, which is more general than a specific machine, such as a Refrigerator).
So for a particular instance of Refrigerator, the calls cm.start(); and rf.start() work the same way but cm might also be a different CoolingMachines object. And that object could have a different implementation of start().
