As you develop new functionality or a new product, you, the designer, are the expert about how is should be used. On the other hand, there’s the practical user, who is going to use your product in everyday situations and should become an expert in the use of your product. Usually, these two persons are not the same. Though as a designer you tend to put yourself in the position of the user, trying to figure out what you would want as a functionality.
In essence, this design process is very wrong. As a designer you are an expert in designing things, and you’re such an expert of your own product, that you immediately know how to use it from the start. You even might think you know which functions you need at which moment. This is usually not something the designer should think of on his own, and this is why the end-user should be considered and consulted all the way throughout the design process. Testing your product in order to get a good design is of utmost importance.
Every person is a unique individual, even single-egged twins aren’t exactly the same. As you grow up, you learn new and different things which influence your interests and the choices you make in your life. These interests influence how you perceive the world around you, what gets your attention and what doesn’t.
The difference in this perception is a big issue for developers and designers. How can you make a program or object that suits everyone? This is something that’s impossible to decide by yourself, in order to find out whether your program is of any use, you’ll need to test it with other people in real-life situations. Sometimes this means that you’ll need to design different versions of your software for different (groups of) people.
The most common example of this is software being in different languages, but this difference could go further than just language. White cars reflect the most amount of light, thus meaning that they are cooler and easier to cool in hot countries, but harder to find in the snow in cold countries. Designing a practical and worldwide useful object suddenly becomes a lot harder!