Its not always obvious to the user on what to do.
Iterative design and evaluation
Evaluate what is being built throughout the process.
Formative evaluation is during
Summarative evaluation is after.i
Evaluation take place within a framework of users their work context the environment within the work and the technology they use.
What do you want to know and why?
Could be:
Understanding the real world
Comparing designs
Engineering towards a target
Checking conformance to a standard.
Some ethical aspects
Let user know aim of the test in advance