Copy-SPSite is an easy way to duplicate SharePoint Site Collections as Todd correctly states. The use case Todd describes is a valid one - but as he as wrote in one of his comments you must delete the source site right after using Copy-SPSite.
GUIDS within the sites are getting duplicated as a result, what can cause you serious problems later on:
Exporting content using export-spweb is going to fail if sites that have been duplicated using Copy-SPSite are within the same content database.
OneDrive Sync Client might fail syncing content from sites that have been duplicated using Copy-SPSite.
There is an issue with deleted items in Recycling Bins of duplicated sites.
Third Party Backup/Restore Tools as well as Microsoft DPM is failing restoring content from duplicated sites.
Please make sure to delete the source site if you consider using Copy-SPSite.
Premier Field Engineer, Microsoft Germany