Why do you "need" office?...
If I didn't HAVE to use MS for some proprietary software and macros, I would use LibreOffice.
My best guess would be the same reason pretty much every other company "needs" MS Office. It's trusted, industry-standard software, ensuring maximum document compatibility both inter-office and with clients.
Yeah, it's expensive, but do you want to be the company constantly telling clients that the document they sent you won't open correctly in your FOSS Office alternative and your employees don't have time to spend trolling forums for a workaround? For many companies, that single instance of lost employee time and the negative image your company just put toward a client just cost you considerably more than the cost of the software license.
It sucks, but in the business world often you're not paying that "business class" price markup for extra features and usability, you're paying for the assurance that out of all the available options this one is going to "just work" more often than the rest, and if it doesn't it's backed by a trusted manufacturer. Because at the end of the day, time is money, and when something doesn't work your company is losing more money than $160 a copy for Office.
Back to the OP, the catch is Outlook, which means you need at least Office Home and Business which typically has a consistent price between most manufacturers. Your best bet is probably Amazon for retail boxed copies or single-machine OEM license keys (a little cheaper, but legally they die with the PC). You can usually get the full retail Home and Business 2010 for about $160/copy, whereas at a Staples or Office Max you're easily paying over $200.