Use Copy Entire Site when you need a full copy of an existing site — for example a development copy, or a starting point for a new full-site package. The copy includes the site’s content and its package associations.
Export and import can also duplicate content, but imports do not keep existing package associations. That often causes trouble later when you try to manage packages on the new site. Prefer Copy Entire Site unless you specifically need an export file.
For a downloadable backup file instead of a live copy, see How to Back Up and Export a Full Site.