Storyboard reference in Xcode, where should we use it?

UPDATE (January 6, 2016): I just want to quickly mention that using Storyboard references is very simple and is going to help you use Storyboards in a much more clean and maintainable way. A good use case for it is e.g. a UITabBarController with multiple tabs. Just create one Storyboard for each tab and in your Main.Storyboard link to those individual Storyboards using Storyboard references. Usage is very straightforward: after creating a Storyboard reference you only need to give it the filename of the individual Storyboard that you want to link to and set the initial view controller within that individual Storyboard. That's it! :)

What follows now is the original answer I gave to @YogeshSuthar's question.

1. this can be used in cases where you are using multiple storyboards in your app. until now you'd have to instantiate view controllers from other storyboards programmatically, seems like now you can just use this reference and create your segue in the storyboards just like with view controllers from the same storyboard

2. yes, you connect one view controller from your current storyboard with another view controller from a different storyboard and you can create a segue between these two

3. yes, this can be used to replace the code that was formerly used to instantiate view controllers from other storyboards programmatically

4. [UPDATE thx to @AlexBasson] Storyboard references can deployed to iOS 8, OS X 10.10 and watchOS 1.

Usage of Storyboard Reference

Well other answer's are correct to my questions.

And I want to post the usage of the Storyboard Reference which is used to open View controllers from another storyboard via segue without writing any code. :)

Steps

1. First create 1 new storyboard name it anything you want(I named it NewStoryboard).
2. Place your View controller(or any View Controller) in NewStoryboard.
3. Give it a Storyboard ID(I have given newVC).

4. Switch back to your main storyboard.
5. Drag and drop Storyboard Reference control in this storyboard.
6. Select the Storyboard Reference control and select Attributes Inspector.
7. Select your new storyboard NewStoryboard.
8. Provide Reference ID as newVC(which you used in NewStoryboard storyboard).
9. Connect this Storyboard Reference via segue from ViewController. Now it will open this VC via segue without writing code. :)

4. Will it work on older iOS version(before iOS 9)?

Looks like maybe not :/

Storyboard references required an app targeting at least iOS 9.0, OS X 10.11, or WatchKit 2.0 for watchOS.

Source: watchOS Developer Library: Adding a Reference to Another Storyboard