The purpose of any computer application is to be used in an effective manner. Often, however, users turn to their own strategies in order to carry out their work. The analysis of this gap, notably the one existing between what ergonomists call prescribed work and real work, is a key element of ergonomics.
The science of ergonomics is, by definition, intended to understand and transform human work practices as to minimize physical or mental suffering and contribute to balance and personal development in business, or other, environments. In the realm of information technology, ergonomics has the particular goal of improving the interaction between a computer and the person using it.
Major usability problems can be identified and understood thanks to the mere observation of user activity, and user testing can be especially helpful in determining what may be lost between the intentions of the designers who create applications and the ways in which these are in fact used in real life.
For more details on how the observation of the interaction between man and the machine is still the best way to adapt the latter to the needs of those who use it, please see the following article (in French) by Laetitia Giannettini.