There are things you can do with a storyboard that you can’t do with a nib. A storyboard lets you create segues between view controllers, and it lets you design table view cells in-place.
There are things you can do with a nib that you can’t do with a storyboard. In a nib, you can create references to the File’s Owner placeholder. You can create multiple top-level views, edit them, and create connections between them. See this answer for an example of why you’d want to do that. You can add external object placeholders (a rarely-used feature).