To learn how to use Photoshop to optimize a photo for printing.
When purchasing professional images from photographers, you will be buying RGB images (stock image downloads are in RGB mode as well). This is because RGB images have a larger color gamut. Therefore, all retouching and compositing work should be done in RGB mode and saved in the PSD format for archival purposes. The PSD format is an excellent archival format because it is non-lossy and will retain all your layers for future editing.
If the image is to be used in print, one must first convert it to CMYK before placing it in InDesign. Once an image has been converted to CMYK, it has lost some of its colors and those colors can never be brought back. This is why one should always keep an archival RGB image for safe keeping.
In addition, images are often used for multiple applications and will most likely be published in multiple media. At the very least, an image might be used for both a brochure and a website. Sometimes, the same image will also appear on billboards or on TV. Depending on the media, RGB images may be required instead of CMYK. This is another reason why your archival image should be saved in RGB mode.