Harold,
I can't quite follow your diagram, but in most cases when a DC motor or something with a winding like a solenoid is involved which is powered using switch contacts, it is customary to protect the contacts from arcing with diodes. When the switch is opened the field in the motor collapses making it generate voltage in the opposing polarity. The diodes are wired so they short that current to ground reducing or eliminating the arcing in the switch. Not sure if that is the whole story here but it looks like it.
Later Ed