Separate personal from business
Makes it possible to have a personal Inbox from your regular BlackBerry Inbox used for business.
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Access to features normally not provided
For example, the ability to access GMAIL labels. Or the ability to do IMAP IDLE. These features are not provided by BIS or BES.
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Keep less important emails non-push
Not everyone wants push email. Less important emails such as those sent to your GMAIL address, can be separated into a separate Inbox that doesn't alert you. (There are ways to configure this with BIS, but you do get better GMAIL integration with GMAIL Mobile, for example)
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Use for Non-BlackBerry Data Plans
With Rogers Canada $5 Vision data plans (10MB) being cheaper and having more data than Rogers Canada $40 BlackBerry data plans (only 7MB), some Canadian BlackBerry users have been using third party email clients on BlackBerries, and the HowardForums thread for non-BB plans on BB is very popular in Canada. (Note: It's possible to use generic data on a BlackBerry, bypassing as BlackBerry plan, but you lose the @blackberry.net push email)
Let's not forget the complement to the GMail @gmail.com client, the Mail by Google client for people using hosted domains with Google. From your BlackBerry: