The IMAP protocol, by default, allows the user to keep all messages on the server. It constantly synchronizes the e-mail program with the server and displays all present emails. All the actions performed on the messages (reading, moving, deleting...) will be done directly on the server.
Because everything is kept and managed on the server, the user will be able to access the e-mails in all server-based folders from any computer in the world (as long as they're connected to the Internet!) and will always find the same content in their e-mail account.
Because there is a quota on the e-mail servers, it is necessary to regularly delete unnecessary e-mail messages from the server, to avoid problems.