There was Layout which is now Notebook and then there's Layout which is different. Which Layout are you looking at?
The original Layout, now Notebook, was/is a 2D non-scaled centralized location for commonly used values and some datum relationships. The most obvious use is to capture numeric values that are identified prior to starting the detailed design process. The less obvious is to be able to magnify small details against a background that would otherwise hide it, since the sketching is not parametric and therefore not tied to any scale. It is the digital equivalent of a hand sketch. A Notebook can have many sheets, like a drawing, and can include comments in the form of notes.
The new Layout looks like a 2D to 3D non-parametric bridge, sort of like a 2D non-parametric skeleton, but in a more user friendly environment. It is required to be to scale, so it cannot emphasize small details, and it doesn't seem like it's an easy way to share numeric values explicitly.
http://learningexchange.ptc.com/tutorial/2280/layout-file-types-and-uses is a little bit of the difference.
