Look at the voltage across the inductor with the diode switch open first and open the main switch. Notice the huge negative voltage generated by the inductor as the inductor resists change in current. This is referred to by many terms such as inductive spike or flyback potential.
With the main switch open, close the diode switch, and now close the main switch. Let the current reach maximum and now open the main switch. What do you notice about the size of the inductive spike? Now the inductor can discharge through the diode without needing to generate a huge potential. The diode is referred to as a flyback diode.
