While the concept of usability is fairly straight forward, differentiating it from usefulness is not always easy. In both cases, it’s about a match. A product with good usability has been designed so that the required interaction follows the way users think about it. Expectations are met, and users are able to accomplish tasks without frustration.
Usefulness, though, is not about the way the interface is designed. Rather it refers to a match between need and product purpose. When they work as intended, products fulfill an identified need in the customer market, so their purpose is inherently useful.
A second kind of usefulness, though, refers to the frequency with which a customer will might use a product or service. A small kitchen knife, for example, can be used to peel an apple, spread butter on toast, crack an egg shell, or even cut the tags off a new pair of clothes. While it might not be ideally suited for any of these tasks specifically, the sheer versatility sends its contextual usefulness so high that virtually every home has one.
Of course, frequency of use and versatility are not always prerequisites for good utility. For instance, one would hope to never have to use an AED device, but it would be incredibly useful in the event of a cardiac emergency. In the case of an emergency medical device, ease of use for a very specific purpose is of primary importance and versatility may actually be counter-productive.
On the other hand, products that fall into the domain of convenience often must need to both do what it sets out to do, and be used often, in order to be widely successful. Products that appear in late-night infomercials provide a good illustration of how hyper-specificity and actual need are sometimes overlooked. For instance, ‘Easy Feet’ purports to fulfill the very specific need of foot washing. To establish Easy Feet’s purpose, the infomercial begins with the familiar black-and-white images of people having a terribly hard time using a multipurpose tool (in this case, a washcloth), for a purpose that it is not explicitly designed for (washing feet). But while the Easy Feet is demonstrably excellent for comfortably washing feet, its hyper-specificity prevents it from having any other purpose.
Perhaps the best example of a hyper-specific product is the EZ Cracker. This large hand-held product has one single purpose: to crack eggs. In this way, it fulfills the first criterion for usefulness: it cracks them very well. Still, the EZ Cracker (and its counterpart, the EZ Scrambler) exemplifies these types of products: one would have to perform this task significantly more often than the average person in order to justify buying, storing, and even remembering to use this tool.
Although we did not actually test them, the interface and design of these, and many other infomercial products appear quite simple, and the use scenario clear. Achieving mass appeal, however, is about more than simplicity when the problem solved is such a convenience. In the end, good products are those that have been designed with both usefulness and usability in mind.