There are four big benefits to storing in a PST.
It may seem as if your e-mail is already on your computer, because that's where you look at it. In fact, however, if you use a Microsoft Exchange server or an Internet e-mail service, most of your e-mail lives on an e-mail server until you move it.
But move it where? Outlook lets you create local data files, called personal stores (PSTs), that really are on your own computer. By using Outlook PST files, you gain many freedoms:
Freedom from nagging If you keep everything in the Inbox, and the Inbox resides on a server, you're going to get a message someday saying that it's time to clean up your act. Sometimes the messages even threaten to stop your e-mail until you do. Store your e-mail in a local Outlook storage file, and you'll be free from this type of nagging.
Freedom from clutter With most stuff stored in a separate location, you'll be able to concentrate on the messages you really need to keep in front of you.
Freedom from bulkiness PSTs can be compressed, which makes them an efficient way to store your messages. Less disk space for them, more for you.
Freedom of movement PST files are portable: You can move them from one computer to another, and you can easily store them on a CD. You can store them on a computer that you share with other people, and you can assign a password to help control who has access to the messages.