The contractor marks changes on a set of drawings during the job. The marked set goes to the owner at closeout. Some specs call these record drawings.
Example: an electrician moves a conduit run to miss a beam. The electrician marks the new route on the as-built set.
For an estimator, as-builts matter on renovation bids. The original drawings may not match the building. Ask if as-builts exist and compare them to the new plans. Check the closeout section too, because someone must spend hours to produce the as-built set.