The main advantage of IMAP over POP3 is the ability to keep the original e-mail stored on the mail server and the ability to access the same account from multiple locations. Unlike POP3 where e-mail is downloaded to the computer or device checking for mail and completely removed from the mail server, IMAP simply downloads a copy of the e-mail. This is much more convenient as the e-mail account can be set up on multiple computers and devices using IMAP and the original e-mail will always remain on the mail server.
In cases where a mobile phone or personal computer was configured to be the primary means of checking e-mail and malfunctions, is lost, stolen, or totally inoperable, the e-mails will not be lost. Set up the account using IMAP on another computer, device or simply check the 1&1 Webmail at http://email.1and1.com/ and find that all e-mails are still intact!
Simply, this means if one were to set up an e-mail account on a mobile phone as POP3, the phone would download all e-mails, removing them from the server. If one were to then check the webmail or home computer for the same e-mail account, there would be no mails viewable as they have already been download by the mobile phone.