The need is simple: The search box sends what you type in it to your search provider immediately. So if they're combined, there's no way to type a url without

The need is simple: The search box sends what you type in it to your search provider immediately. So if they're combined, there's no way to type a url without your search provider knowing what URL you typed. Now maybe you're OK with that. But a lot of users are not (including non-technical users who don't even realize this behavior exists; once you explain what the unified bar does to them they become _very_ unhappy).