While not exactly illustration, photo retouching is a skill that utilizes many illustrative principles. In this case, the products have to be cleaned of all their dust and scratches, and I've even found myself having to completely repaint sections of devices that were too damaged or dirty to realistically salvage. Likewise, many products or magazine shoots are very specific about the look and feel fitting a certain "brand". Color, composition, and values all need to be on point, and they images aren't always shot that way. Many times that work needs to happen in post. Retouching can make your good photos great. It can make your final product shine.