Poco General FAQ - Why compress mailboxes?

Q: Exactly what is the reason or need to compress the mailboxes, just to take-up less space or____ ?
Compressing a mailbox will both reclaim space consumed by deleted messages and will also re-build the index of the mailbox. This can result in better performance and is also necessary after certain script operations or if you have configured the mailbox index to include custom columns.

Austin has provided a more detailed explanation of the compress process.

Poco will automatically compress the mailboxes from time to time but you can initiate the action yourself by selecting the mailbox in the mailbox index and selecting "Compress Mailbox" from the "Mailbox" menu. To compress all your mailboxes at once use the "Compress All Mailboxes" command on the "Mailbox" menu.

Script writers can use the CompressMailbox to compress a mailbox from within a script but please read this cautionary note first!