For starters Outlook isn't web based. So you don't need to be online to use it. You need to be online to check mail, but many of its other features (depending on whether you mean Outlook Express or MS Outlook) can be used off-line. Calendar, Address Book, etc..
Anyway, you obviously don't have to worry about storage limits like you do with web based email services, since all mails are stored in your hard drive. As long as there is hard drive space you can store emails.
Outlook has more diverse filtering mechanisms, and you can check more than one email account at the same time.
For more specifics you need to tell us which Outlook version you are talking about. Outlook Express, or Ms Outlook.
Since they have radically different features.
And you need a special type of email account, POP3 or IMAP to use Outlook.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.