Well until Xcode 6 (I think). Neither xibs nore storyboards were indexable. Meaning if you wanted to find something related to your layout, you would had to search through xml. Also much easier to refactor typed code than xml ;)
Wait, why would you fix your layout bugs by searching through XML? Why not just use interface builder?
Because you'd have to manually go through maybe > 39 scenes spread across 10 storyboards manually keep track of what's changed and find the next outlet you want to modify.