The archive step is used when you want to hide (unpublish) the page from live site but still keep it for some reason in the content tree. E.g. content archiving. This step should be at the end of the workflow. Usually, the step before Archive is the Publish step. So the page is in published state - available on the live site, before it gets published. In many cases this step is not used, but it is one of the mandatory workflow steps to ensure correct workflow cycle - Edit, Published, Archived are the mandatory workflow steps.