My apologies, from what I understand that you are wondering why there is an option to add a timeline to an existing state?
If I take my previous example of the display controller, we have the shown and hidden states. As this controller is automatically added to every project, it has different properties of the root node already being changed depending on which state is activated. I have added a screenshot below to explain this further, but each property has a transition time that can be changed from instant, fast, normal or slow.
Editing these values and selecting the shown and hidden states simultaneously will give a visual representation as to the difference between each transition. As seen below -
These transitions are actually timelines being set up automatically behind the scenes and the way to access them is to right-click on the state and click add timeline! I have added an example which first shows the fast timeline, from the shown to hidden state and then goes onto showing the slow timeline from the hidden to shown -
So in answer to the question, these embedded timelines are used to create unique and edit existing transitions from one state to another.
Hope this helps