Using pop3 rather than imap allows all mail to remain on the (Virgin’s) servers regardless of what you do with mail ( any message) via your email email client; whereas imap deletes a message from the server as soon as you delete that message via your imap client. imap reflects any change to a message status (i.e. deleted) on one on device across all imap enabled clients. Delete a message on device one and it’s deleted from all other devices.
You can have pop3 on every device you use to access mail, and none of them (ought not not) do anything to the mail still on the mail servers. Thus you by using pop3 you have form of online storage, somewhat similar to having a cloud storage area, and thus accessible from any device. Deleting a message on one device doesn’t remove it from other pop3 enabled devices/clients for that email account.