When we think of colors, we often think of many different shades of each primary color. Many people would assume that the one exception to these color variations is black. After all, we think of black as being absolute darkness, and expect it to appear this way when printed on a document as well. However, black that is used in full-color (process) printing is transparent, like all process inks, and cannot cover ink or paper as thoroughly as you may like.

Although using an opaque black ink, as opposed to the standard transparent ink may seem like a simple solution, but it would cause adverse reactions to other color or high-res images that contain black ink.

Rich black, is an ink mixture of solid black over one or more of the other (CMYK) colors, resulting in a darker tone than black ink alone generates in a printing process.
A typical rich black mixture might be 100% black ink and 40% of each of cyan, magenta and yellow inks. The colored ink under the black ink makes a "richer" result the additional inks absorb more light, resulting in a closer approximation of true black. While in theory an even richer black can be made by using 100% of each of the four inks, in practice the amount of non-black ink added is limited by the wetness that the paper and printing process can handle. Wetness is not a problem with laser printers, however, interesting effects can also be achieved with a laser printer by combining 100% black and 100% of cyan, magenta, or yellow.

So when designing pieces that have large black areas or borders rich black will provide a much more striking results.