Did you every see a spark from a switch you turned off. Imagine if
you switched power on and off over and over again. The switch would
wear out. A problem for some motors is that their insulation was not
rated for switching on and off so much. DC doesn't change that much,
so you can make it cheaper with less insulation. Motors rated for
PWM or SCR are beefier to support high switching rates. Otherwise
they are similar.
