While Apple boldly cut the cord with iOS 5, moving from iTunes to iCloud, it looks like they're wisely leaving some account management power in the veteran desktop app -- namely the ability to manage iCloud associated devices.
With iOS 5 on your iPhone, iPod touch, or iPad, you can choose to enable automatic downloads -- so any app or iBook (or music in the US) bought on iTunes on Mac or Windows PC, or on any iOS device on your account, is pushed out to every other client on that account. Buy Pages on iPhone, it automatically installs on your iPad as well. With iOS 4.3 and iOS 5, you can also call up a list of previous app (and music in the US) purchases and re-download them.
So seeing exactly which devices are on your account, and being able to remove them if you have to (buh-bye, ex!) is a great feature for iCloud to have, and it makes sense for it to live in iCloud.
Moreover, since you can only change the account any device is registered to once every 90 days (to prevent everyone from logging in to every friends' account and downloading everyone else's content without paying for it), iTunes will also show you how many days you have left until you can switch accounts again.
If you only have one device, it may not be a big deal. If you're managing a family, or your account status is as complicated as your relationships, it may matter to you more.