It is going to be run on 3V (2 x 1,5V cells), but it is nice that it stills operates when the batteries goes a bit low. The 2,32 V value was just the lowest I could get it to work (with the motor) when testing it on my switching power supply.Seems dubious to me. On a 2.3V supply the diode will limit the gate voltage to 1.8V which is hardly enough to switch even a logic-level MOSFET.
Probably it's limiting the motor current and thus preventing the supply dipping as the motor starts.