An As-Built drawing of a building is an accurate depiction of the building AS IT IS. You see, a building does not always get built according to the architectural design. There are changes along the way and the plans are generally not changed to reflect those revisions.
Not just during the construction, but through the years owners add walls and reconfigure offices, closets, bathrooms and more. Those changes are seldom reflected in revised plans. So most property owners do not have accurate plans of the buildings they own.