This is not the pin that locks the undercarriage from retracting (damn things!- terribly embarrassing to take off with one in........so I'm told). Those are only attached to each leg by engineering overnight to lock the gear down, and are checked removed before first flight and placed on the flight deck. When a pushback tug is attached to the nosewheel, a different pin is inserted in the leg to lock out the nosewheel steering. As the pushback commences and the engines are started and the hydraulics fully pressurised, you do not want the nosewheel to turn and break the tow bar, and possibly damage or break the pushback crew. Once push complete, towbar disengaged, the pushback crew will remove the pin to activate nosewheel steering, and walk away to show clear of obstructions, and hold up the pin so the pilot knows he has nosewheel steering. Forget to remove the pin and the steering tiller won't move. No steering possible at all.
All very pedantic and precise, but must be followed to the letter. Sometimes pushes commence before brakes have been released (removes leg), and tugs detached without brakes applied again (causes lots of shouting and arm waving). People have attempted to taxi with tug still attached, with the pushback crew still attached by headset (if you get airborne with them there, they do tend to bang against the fuselage a bit, until the cable breaks, and require replacing). More can be bred though- there seems to be a steady supply.