POP3 automatically retrieves your e-mail from the server. You can read it and respond to it at your leisure, even when you are not actually connected to the network. The next time you connect, all the messages that you have written are sent and all your new mail is downloaded. This works wonderfully if there is only one computer in your life. However, that is no longer the case for most people.
Most people have computers at home and computers at work.
They want to be able to read their e-mail in both places. The way they usually do this with POP3 is to set up one machine so it deletes mail from the server and the other so that it does not. You must have at least one client machine that deletes mail from the server or it will just accumulate there and eventually choke. Let's suppose that you designate your office computer as the one that pulls off all your e-mail from the server. Then you will have a complete set of e-mail at the office.