The reason is so you or someone else can't reach around and turn the key off and lock the steering wheel while you are moving down the road. They wanted to make it a little bit more difficult for the manually equipped trucks to lock the steering. The auto equipped trucks with the lever on the column have it built into the shifter. You can't take the key all the way out and lock the steering column until the tranny is in park.
I just traded for a junk Aerostar van that I am going to scrap, and it also has the little button. It's a automatic, but the lever is in the floor with no interlock to the column, so they used the little button on that application also.
I would leave it on there for safety reasons.