The "Steering Column/Transmission Shift Interlock System" also prevents you from turning your ignition key all the way to OFF-LOCK while the car is moving straight ahead. GM was very concerned that with a locking steering column, a few people might turn off their ignition key and lock the steering wheel even though the car was moving. So they invented the backdrive system as a safety feature. You can always turn the engine OFF, but you cannot lock the steering wheel and remove your ignition key unless automatic transmission vehicles were in PARK or manual transmissions had to be in REVERSE.)
Locking the column while moving may seem remote but people sue over less obvious things.
Just a bit of trivia which may or may not be usefull. There are five positions for your ignition key in the ignition lock cylinder. From full CW to full CCW:
START, RUN, OFF, OFF-LOCK, & ACCESSORY.
Some header systems interfere with the backdrive linkage so a lot of owners remove the linkage and permanently lock the lower column lever full UP (out under the brake master cylinder) and at the same time the shift bowl near the dash (full CCW) is locked in position as well.
One other thing, some vehicles (I am not sure about your Chevelle) use the backdrive system to rotate the lower lever on the column which rotates the shift tube inside the column which actuates a switch which controls the backup lights and the park/neutral start. That switch may be mounted on top of the steering column tube down under the dash. If your state has safety inspections, you may need the linkage to operate the switch. (Other systems may have the park/neutral start switch mounted on the shifter in the console and would not be affected by eliminating the linkage.)
This paper (in Microsoft Word format) has several drawings that help explain the above.