I've seen layouts but don't really understand what is the difference if you use them in a assembly or how do they help...
i have not used it much, but is is useful for top down assembly. Its tricky but very useful for large assemblies.Ali's link will give you a better idea.
We use where part size's are controlled inside the assembly, an assembly layout can be used for design tables and base sketches of parts are converted sketches from layout (top down as said above).
Layout tool is useful if you doing intensive with planar mechanisms.
It is fastest way to changing dimensions and see results quickly.
It's useful if you must working with pipeline, tubelines in space and if you must design for limited space.
Add to that, sometime it's useful for mechanisms involving hydraulics and such, planning a layout for an excavator mechanism is much easier to edit with the Layout tool.