Electric motors are electrically noisy, the sparks commonly seen at the brushes are a source of radio frequency interference [RFI] that can cause problems with both the host controller and other systems [have a look at this video of brush arcing]. Historical fact; Marconi used an arc to transmit the first radio signal across the Atlantic.
A new motor and a properly designed motor controller may go on working for years without suffering problems, but over time the arcing and RFI will get worse, and there is a statistical probability of an energy spike of just the right parameters to blow a MOSFET.
Fitting motor noise suppression components, and following these installation guidelines will significantly improve the long term system reliability by reducing the chance of MOSFET latch up and failure.
Have a look at this short video to see examples of a motor with and without noise suppression…..