Question about the role of specific element in a circuit (H Bridge)
They are there to actively turn off the TIP120's, so that they turn off faster and in a more controlled manner.  Try simulating your circuit with higher PWM frequencies (I'd start at 20kHz) and see what happens.  Keep pushing it until you see shoot-through current (current spikes on the power line because a TIP125 and a TIP120 are both on at the same time) -- then put those transistors back in and see what happens.