If you have a Pop3 account it means your emails are stored on our server until you access them on your PC or Mac, when you do that it downloads the emails to your hard drive, clearing the server out in the process. The benefits of this are that you’ll have offline access to your emails and won’t have many issues with mailbox space on the server, additionally it means you’ll be able to store as many emails as your hard drive can hold.
IMAP on the other hand syncs your emails on your PC or phone with the mail on the server and stores a copy on both devices. This means that meaning if you delete something on one it will also be deleted on the other. The good news here is that if your device crashes or is lost, you can still retrieve any emails left in your IMAP account, from our server. You can read more into it here.