it should be used for (partially subjectively) determining whether the system behaves as expected or not and to be able to quickly name the "misbehavior"
e.g. first task should be easily solved by everyone in less then 1 minute (and without any explanations from other people!); when I clicked to the "Easier task" I really want the next task to be easier; when I am an experienced programmer I would like to have loops and conditions already during the first session etc.
or even more high-level "expectations of a user"
it may be also useful to draw a "user finite automaton" (the application states should probably correspond to this)