Comment by direwolf20

POP is a simple mail transfer protocol (hehe...). It supports three things: get number of mails, download mail by number, delete mail by number. This is what you need to move mails in bulk from one point to another. POP3 mail clients are local maildir clients that use POP3 to get new mail from the server. It's like SMTP if it were based on polling.

IMAP is an interactive protocol that is closer to the interaction between Gmail frontend and backend. It does many things. The client implements a local view of a central source of truth.