Archive PDMs provide a snapshot of the structure of your database at a point in time to allow you to determine model changes since that time when updating your database. When comparing your model directly with a database or script (and not with an archive PDM), some differences (particularly around renamed objects) can be lost, leading to more drop/creates in place of alter statements.
Archives are created by default when you generate or update your database (using the Automatic Archive option), and can be created manually at any time by clicking , and selecting Archived PDM (bin) or Archived PDM (xml) in the Save As Type list.