Imagine a Photoshop world without the ability to select. Impossible. There would be no way to work on parts of an image. You would be forced to make changes on the entire image or not at all. Basically, selecting is comparable to creating a mask for a layer. You want the easiest way possible to isolate an area of an image. In some cases, making an edge drawing to create a selection or a layer mask takes a great deal of time. However, by isolating a channel of your image, you may simplify the selection process. In this article, Harold breaks down the important application in Photoshop that allows us to apply a channel as a layer mask–Image > Apply Image.
Read Using Image Apply Image.