A lookbook shows how the pieces of a season belong together. Where a product page isolates one garment, a lookbook image shows a whole outfit, in a location or a studio set that carries the idea of the collection. Brands send it to retail buyers and the press before a season, and use the same images later in their own shop.
A lookbook is read as a sequence, so consistency matters more than in single product images: one model or a small cast, one light, one kind of setting. It does not replace the plain product image, which states colour and cut without a mood. The two work together, one to sell the idea and one to describe the item.