The so called POP3 workaround is a method using POP3 to effectively manage messages for a single mailbox using multiple computers or devices. Unlike IMAP, which actually synchronizes with messages on the server, the POP3 workaround temporarily leaves messages on the server ensuring that email is available to be downloaded by the same user on additional devices.
This practice is suitable for the user who needs to check email from home after hours or on weekends. By configuring Outlook on both the work and home computers to save copies of email on the server for a period of time and to delete messages from the server when messages are selected to be deleted in the client, users will see the same messages on both machines. In addition, the deletion preference prevents unwanted messages from being downloaded multiple times.
In contrast to IMAP, the POP3 workaround does not provide a true single inbox on multiple devices. In short, POP3 users will see separate inboxes on each device, each with copies of the same messages. In addition, while the POP3 workaround suffices for a limited number of similarly configured devices, adding a new device throws a wrench in the gears. Since the new device will only be able to download recent messages from the server, old messages must be imported from one of the other devices. Using IMAP, on the other hand, new devices synchronize with the server meaning message imports are not required.
More info on POP3 and IMAP can be found in our IMAP vs POP3 discussion.