A type family generally contains three variations on the regular face: italic, bold, and bold italic. However, many families have been designed to include variation in weight from ultra light to ultra black; variation in width from condensed to extended; multiple character sets, such as small capitals, titling capitals, swash capitals, old style figures, alternates; and more.
The variety enables you to achieve just the look you want and allows for a good deal of flexibility. For example, it is often necessary to make a given amount of type fit into a predetermined amount of space on the page. When space is an issue, a condensed or extended version of a typeface can be a real life-saver!