It is a misconception to think that all the emails in your Gmail account are safe and will be there forever. In the events that Gmail is down, or for some reasons, you were banned by Google, you won’t be able to access all your emails and all of them will be gone forever. To prevent such incident from happening, you might want to backup your mail in your Gmail account to your system. Come to think of it, you backup your system regularly because all the data and files in your computer are important to you. If your emails are important to you, shouldn’t you do a backup too?