Layouts serve as an interface between the desiigner and the fabricator....
The design engineer designs his circuit and he needs to pass on that info to fabricator..
Fabricator cant understand the normal schematic that we draw.....
So, the schematic is converted into the layout which the fabricator can understand
fabrication process involves proper doping in selected regions of the die to account for all the transistors and we need to clearly demarcate the area where to implant the dopants to form the gate,source drain and all that stuff with interconnections too... That drawing is layout shortly...
Layout gives the area overhead and related constraints and also related issues like place and route can be optimized.
Schematics does not give the fab anything related to the Layers that has to be fabricated.
It is the layout that goes to the Fab for fabrication.
Layout u can say is the drawing thatrepresents schematic in a way that it will fulfill all the functionality and the parasitic related issues when fabricated.