Why do we need psd layouts to create websites?

Question

Layout

As I understand it, the layout designer is given a site layout made in Photoshop, and he simply turns it into an html file, but what is this for? Why create this layout at all, why not create a website right away? Why can't the layout creator create the site as well? The layout designer is given the icons of this layout, or does he have to re-create them using some svg, or look for font-awesome? If a layout designer is not given a layout, but simply told to create a website, can Photoshop or its replacement be useful for icon design?

Answer 1

As I understand it, an engineer is given a drawing on paper, and he builds a house on it, but what is this for? Why blueprints at all, why not build right away? Why can't an architect build a house? An engineer is given floor plans, and he has to build them out of bricks? If an engineer is not given a blueprint but told to just build a house, can archCAD or its replacement be useful for floor plans?