During a mapping session, it's is often very nice to refer to previous mappings for that same field. As we move from Wave to Wave, we have the ability to Export the Mappings into the System Types from the Vertical View of the Map / Sources. These mappings can then be re-imported in mass to start the next wave if we encounter the same source again.
However, sometimes it's not appropriate to import ALL the mappings for a source, but during the mapping session we might want to see what was done with that field in previous waves. Having the ability from the Vertical View to open a page that shows all the archived mappings (from System Types) for that particular Table/Field is a really nice feature. This would generally be a read-only view, but could possible include an IMPORT button at the record level so if you see a mapping you like you could just click the button to import that one mapping for this one field.
I noticed that in the mapping sessions at 3M that folks would often have several Browser Tabs open and spend a lot of time bouncing back and forth to the other Waves to review "what we did in Wave 1, etc". It was a simple change to give them a drill down to the archives so they could quickly see all the historic mappings rules from those previous releases. Very handy.