I used to know the answer to this.

I think there were only wheel straps on the front wheels. If that were the case, then the parking brake on the rear wheels would add to the stability of the car at least a little. If the rear wheels were strapped, the un-strapped front wheels would not have any brake to hold them.

Be careful out there; vary from the recommended pressure by even half a psi, and this could be your car crashing.