The Packed Binary file is created by the Export-MapiPackedBinary cmdlet and is unique to PowerMAPI.
The Packed Binary format is non-editable format where each MAPI property for a message is stored sequentially in the file with descriptor of each property. This format ensures no "white space" will bloat the file and provides for very fast reading of the file in import. Furthermore, the data in the file is not compressed which can allow search operations to work against the text embedded in the file format. All recipient and attachment information is also saved in the Packed Binary format providing search indexing and abilities within that data as well.
As mentioned in some of the remarks for MSG file cmdlets, PowerMAPI's Packed Binary format is much more efficient for export, archival, and import routines and by its design does not lose ANY message attributes. Outlook's MSG format is not able to preserve several key message properties like: EntryID, Modified Date, Message Size, etc.
PowerMAPI also provides the Open-MapiPackedBinary cmdlet for cases where a Packed Binary files needs to be inspected, but not imported.