As is noted on the "ENGL 1113 Diagnostic Writing Exercise" page, it is helpful for an instructor to have an idea of the students' existing capabilities before beginning to teach them. An understanding of what students are already capable of doing is necessary to be able to effectively gauge the level at which instruction needs to take place to keep it within the students' reach (if with a certain amount of healthy cognitive stretching) and still meet the end goals of the course; both point of origin and destination are needed to chart a course.
To that end, the diagnostic writing exercise, an in-class assignment in which students are asked to look at a short piece of technical writing and use it to address the following prompt (taken from the assignment sheet for the exercise in the Fall 2014 term): "Assuming that [the provided model] can be taken as a model of technical writing as a whole, what features of form, content, and structure are most important in technical writing? What assumptions about its readership is technical writing likely to make? Ground your assertions about technical writing in specific details of the attached document, being sure to explain how those features serve to support your assertions."
The diagnostic writing exercise will not be graded, but it will be used to drive instruction. Student performance on it is also likely to affect perceptions of professionalism, so earnest performance on it is advisable.