I know you can search from the navigation bar (and keywords were there even in the Mozilla suite, weren't they?), but the difference from the search field is that the search field shows suggestions, which means that it has to send what you're typing to your search provider as you're typing it.

The navigation bar only searches locally (the history and bookmarks on your disk) and doesn't send anything anywhere until you press enter.

I was under the impression that this was considered a feature and the reason for keeping both fields even though they could easily have been unified long ago.