POP3 is a collection method.
When accessing emails using POP3, the emails are downloaded on to the device you're using to check your email.
This removes them from the mailbox held server side.
Whilst the use of POP3 does result in lower disk space usage, it also means that other devices accessing the same mailbox won't see emails that have already been downloaded.
IMAP is a synchronisation method.
When accessing emails using IMAP emails aren't downloaded to the device used to check email. Instead what's held in the mailbox server side is sync'd with the device.
Whilst IMAP does result in a larger amount of disk usage, it does mean that multiple devices can connect to the same mailbox and see all the emails, and the state of emails (read and unread) and the contents of IMAP folders is consistent across all devices accessing the same mailbox.
If we take some common scenarios, we can establish whether it's best to use POP3 or IMAP.
"I want to be able to send and receive emails from my computer, my phone and my tablet, and I want everything to look the same on all these devices." This dictates the need to use IMAP.
"I'm happy to use one computer/device to collect emails, and I don't want to use a lot of disk space (which may dictate the need to upgrade my hosting)" POP3 is a good fit in this scenario.
"Both myself and this other person want to be able to access the same mailbox and see all the emails" IMAP is best to use here.
