According to wikipedia, Windows 7 Ultimate has many more features than professional and yet it costs considerably less.
Windows 7 professional, which costs considerably more, has less features and doesn't have even a single feature that ultimate doesn't have.
When would someone buy professional instead of ultimate?
Short answer: there's little reason to buy Professional as long as Ultimate is available; Ultimate is only $20 more and has every single feature offered by Windows 7, including some key features that you might regret not having.
Longer answer:
Whereas Windows Vista had a criticized lineup, with lots of versions and confusing overlaps, Windows 7 has strict super-set SKUs (no venn diagrams) and essentially only comes in two retail versions: "Home Premium" ($200, upgrade $120) and Professional ($300, upgrade $200).
There are two somewhat more obscure versions: "Starter" which is a dumbed-down version which is only available when packaged with low-end PCs (notably netbooks) and then the souped-up "Enterprise".
"Ultimate" is a retail version of "Enterprise", the latter only available with volume licensing; there is no technical difference between the two. The exact differences between Pro and Enterprise are clearly described on Microsoft's enterprise page: http://www.microsoft.com/windows/enterprise/products/windows-7/features.aspx, in my paraphrasing (apologies for my humor, duly labeled):
In other words the Enterprise additions are, as expected, enterprise-oriented, e.g. security, remote features, vm stuff, etc. Nothing you would typically care about unless corporate MIS makes decisions for you. With one notable exception, the already mentioned MUIs (multilingual user interfaces). This feature was considered enterprise by MSFT because of virtual machines - e.g. enterprises want to be able to set up reference operating system images that can subsequently be reused in multiple language regions. It turns out that in our global world, many individual end users find this feature useful. But you might like the other features as well. I suspect some people might like the the BitLocker capabilities (in particular with respect to USB storage devices).
The retail pricing difference between Ultimate and Professional is minor ($20) so for an advanced user there's little point to even think about it. Personally, I'll pay the $20 to just avoid kicking myself later.
The Microsoft store is a bit deceiving. It shows the Professional Upgrade for $199, Ultimate for $219 and Professional for $299. However, if you do a bit of digging(click on the Windows 7 product) you find that the Ultimate Upgrade is $219. A full retail copy of Ultimate is actually $319. So, Ultimate is not cheaper than Professional, it is about $20 more either way.
If you look at the comparison Ultimate has everything that Professional has plus the additional features:
The only reason I can think of is the End Of Support: 2020 for Pro, and 2015 for Ultimate. I.e. you receive 5 more years of updates and patches.
Pro versions are usually targeted to business customers, who do not upgrade often.y
