Layouts = included function from 3.5, what's about the plugin "layout"?

While using a theme, i could see the power of layouts.
Now i try my first own theme. What is the situation now with these layouts?
They are included since version 3.5? Is this include the same that the plugin does?
Is there any reason to install this plugin?

The layouts plugin is a completely different story and has nothing to do with the layouts field.

Yes, got it. It means to choose a layout for a page itself like having a sidebar or something like this.