What is Drawable.getConstantState used for?

Question

I use Drawable.getConstantState to compare two drawings, as mentioned here , but could not understand its purpose. In docs, its value indicates that

Return a Drawable.ConstantState instance that holds the shared state of this Drawable 

What is the general condition?

Also Drawable.ConstantState

 is used by Drawables to store shared constant state and data between Drawables 

So, I do not understand what is the general condition here and what data is stored? And how it helps in comparing the drawings.