POP3 is designed to allow users to download all email off of the mail server to their mail client without leaving a copy on the server, or download all email, but leave a copy on the server.
IMAP is designed to allow users to access email from different devices by leaving all email on the server until read, when a copy is downloaded to the user and a copy is left on the server.
With both protocols, email is deleted from the server when the user deletes it in their mail client; some Apple mail clients don't handle that very well.
These links provide some additional info: http://www.howtogeek.com/99423/email-whats-the-difference-in-pop3-imap-and-exchange/ https://www.howtogeek.com/395873/how-to-configure-mail-settings-for-iphone-and-ipad/ https://www.imore.com/how-to-manage-mail-settings-iphone-ipad