Portfolios have two main uses, both of which involve evaluation.
Portfolios can be used to demonstrate the quality of a person's work for hiring and promotion purposes or for purposes of passing a course of study. The same portfolio should not be used for both purposes. The summative evaluation judges the outcome of one's work, while formative evaluation seeks to identify areas to be improved and to suggest possible ways to make those improvements. There is risk-taking involved in the latter as it takes a much more vulnerable perspective on one's work. In general, the Teaching Portfolio is most often used for summative evaluation, that is, for hiring and promotion.