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Freewheel Diodes...
I had that exact question a few months back...
The current in an inductor has momentum. A freewheel diode allows the current to harmlessly go in a loop when a switch is suddenly open. Without them, it would destroy the mosfets when they switched off.
The motor has some inductance. Once the mosfets get switched off, the current from the motor would try to pile-drive its way through from drain to source. The voltage would be like 1000's of volts for an instant.
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