Comments from feedback form - "If Opportunity has been on Mar..."
Fragment of a discussion from Comments:Mars Opportunity rover passes distance milestone
Well, they set a "prime" high priority mission for the rover to accomplish, with the optimistic intention to carry out further exploration beyond the designated lifespan. It's more technical than anything and most spacecraft function past their lifespan, such as the Voyager program probes, which continue to function after their launch in the 70's.