Portfolios have many benefits; perhaps the greatest benefit is that with portfolios, student work/projects (as parts of the portfolio) correspond to theories of writing and learning and to your attempts to "teach/implement" the theories. Since most composition teachers now see, practice, and teach writing as contextual, non-static, and process-oriented, the portfolio model reinforces such theories of writing.
Since portfolios must include a variety of materials (most include student revisions, teacher notes, peer-review notes/worksheets, journal entries/reflections, and an end-of-the course summary) and students must be actively involved in the portfolio process (by making journal entries, writing a learning summary, writing about their selections), students generally come to see writing and learning as interrelated processes.
In addition, portfolios allow teachers to see (through the variety of portfolio selections) students as individuals and as learners!
History and Philosophy of the LRO