I believe the idea was for this capability to go along with the ISS as a life-boat contingency. That is: shuttle is damaged on launch such that a safe return is no longer a sure thing, so the crew links up with the ISS, shuttle attempts an automated return (or ditching), and the crew hang out while another shuttle is prepared to fetch them.
This is also why all but one post-Columbia mission either went to or was within range of the ISS. The only mission for which this wasn't true was the final repair of the Hubble, and for that they had a second shuttle on stand-by for a potential rescue launch on short notice.
This is also why all but one post-Columbia mission either went to or was within range of the ISS. The only mission for which this wasn't true was the final repair of the Hubble, and for that they had a second shuttle on stand-by for a potential rescue launch on short notice.