If the default playing size is within the dotted area, why the rest of the area?
I think you're getting a little confused: the dotted area is the size of the window and the rest of the area is the size of the layout.
If you want your game to be able to scroll, you usually have to have a bigger layout than window area!
By default the layout is bigger than the window because we got tonnes of "how do I make the game scroll?" questions in Construct Classic which were all people who hadn't made the layout bigger than the window. So C2 makes it that way by default. It has no effect at all on games that don't scroll.