Compress Outlook PST file email items such as: messages, contacts, calendars, appointments and other email based information when they becomes over from their size limit. Large Size of Outlook Email files sometimes lead various issues and corruption are some of them. So Compress technique is unique way of dealing with large size Outlook email files through which we all can reduce the size from over size Outlook Email files.
You can also check how to compress PST file manually using the link:- http://migrateemails.com/2016/09/14/compress-pst-compact-pst/