The option to archive a document is to help an applicant organize their "Document Tracking" section.
Most use it to clear documents they end up not using or documents from past years.
Some documents, such as LOR's, won't have the option to be "Deleted", therefore, the "Archive" option helps clear those documents.
You can view the documents you've archived by going to the "Document Tracking" section and clicking the "Archived Documents" tab. In the Archived Documents tab, you can restore any archived documents by clicking the restore icon.