Because you want to be able to effectively place images into your word processing and page layout documents, and you want them to look good. You want to get images from the Internet and other sources and use them in various ways. You want to place images on your web pages, in your computer presentations, etc.
Text documents have extremely small file size.
There is a constant trade-off between image quality and file size . You need a special program, called an Image-Editing program, to open images, manipulate them, and convert them to different formats. With Raster images, each pixel contains specific information on it's color. Each pixel can be only one color, but many pixels can be blended together to simulate lots of colors. This is why you should make your images as few colors as possible.