Because iCloud is typically used to link apps, browser logins are now being treated as unusual enough to warrant an alert by themselves.
These alerts wouldn't prevent hacks, but they would provide an earlier indication that an account had been compromised, making it easier to stop hacks in progress and potentially trace back whoever is trying to break into the account. It's also an important sign that Apple is turning new attention to iCloud, which has many in the industry have seen as a neglected part of Apple's otherwise seamless infrastructure.
It's also an interesting move in the run up to Apple's event tomorrow, which many expect will bring new
features to iCloud.