Considering Purpose and Audience
We typically think of "values" as being personal matters. But in our writing, as in other areas of our lives, values often become matters of public and political concern. Therefore, it is important when we evaluate to consider why we are making judgments on a subject (purpose) and who we hope to affect with our judgments (audience).
Purposes of Evaluation
Your purpose in written evaluation is not only to express your opinion or judgment about a subject, but also to convince, persuade, or otherwise influence an audience by way of that judgment. In this way, evaluation is a type of argument, in which you as a writer are attempting consciously to have an effect on your readers' ways of thinking or acting. If, for example, you are writing an evaluation in which you make a judgment that Mountain Bike A is a better buy than Mountain Bike B, you are doing more than expressing your approval of the merits of Bike A; you are attempting to convince your audience that Bike A is the better buy and, ultimately, to persuade them to buy Bike A rather than Bike B.