Microsoft Outlook uses a database format called a PST file.
The PST file has been around for a very long time. Decades, in fact.
As users receive email in Outlook, the size of their PST files grow. The larger it grows, the greater likelihood the PST file can become unstable and the data within it corrupted.
PST files also have maximum size limitations of around 10-12 gigabytes.
Repairing a corrupted PST file is no laughing matter. It can take hours, and data loss is permanent. Imagine being told that all of your email for the last four years are simply inaccessible and gone. Not a good conversation.
These conditions forced users of Microsoft Outlook to create multiple PST files to manage their data and spread the risk between PST files.
This week, I migrated company that relied on multiple PST files to manage their data to Google Apps. In Google, every user starts with 30gb of storage space and suffers none of the same limitations. This allowed me to consolidate a user with 4 PST's, for example, into just one mailbox. This simplified things for the end-user and made it possible to see all of their folders anywhere they go.
Anyway, as a tech professional, it makes me happy to ditch using PST files and extending capability to users where they've been limited in the past.