The concept of protection diodes across a fan/motor?
The problem comes when you try to switch the load off.  

You don't show a switch in your circuit, but if you had one say in the low side of the fan and you switched it off, without the diode there would be no path for the inductive current except for the parasitic capacitance at the switch node.  

Motors are inductive loads and an inductor can't change its current instantaneously. V=L*di/dt so the V can spike extremely high if the current is forced to change quickly.

By putting the diode across the load you provide a path for the current to continue, with a small voltage across it to discharge the inductor slowly.