This is a great question. Portfolios can be used and seen from many different perspectives. For this reason, portfolios can offer many different kinds of information. One of the points of information that portfolios offer is a summary of a person's work. This is especially true if the portfolio is based on a chronological point of view. Let me give you a few examples.
First, a chronological framework can show how that person has grown, changed and developed. From this point of view, a portfolio is a selective autobiography of a person. This can be very useful in any evaluation (even self-evaluation).
Another kind of information that portfolios can give is the formation of a person's thoughts and influences. If there is change in a person work, then this might be due to some new formation of ideas or influence. In light of these points, a careful examination of a portfolio can yield a lot of information on a person.