Layers are useful for several reasons:
In an interior design context it would enable you to show a customer several finishes on a design at the click of a mouse – ‘Here’s your kitchen built in rosewood – a few clicks – and here’s your kitchen in beech’.
In an architectural situation it enables you to show each stakeholder in a project the relevant aspect of the job that they need to see without the confusion of information that is irreverent to them. Also it enables you to take the roof off when you need to work inside.
On the engineering side, layers in combination with the Drafting Pallet gives you endless possibilities for showing variations of assemblies.
Good layer management speeds up you drawing and just makes good common sense.