Wikipedia: The National Register of Historic Places (NRHP) is the United States federal government's official list of districts, sites, buildings, structures, and objects deemed worthy of preservation. A property listed in the National Register, or located within a National Register Historic District, may qualify for tax incentives derived from the total value of expenses incurred preserving the property.
this means that certain landmarks, etc. have proved their importance to the nations history, that they have to be preserved for all posterity. if a landmark is not listed, then it is not subject to protection, say another building, road etc. needs the land - it will not be automatically saved.