A career portfolio is a good way to keep track of your achievements. It is a single place that you can store all the paperwork to prove that you have actually done the things you claim on your resume. In fact, as you progress in your career, you should maintain one full portfolio that serves as an archive of all important career related materials, and then a pared down portfolio which you actually show to potential clients or employers.
Having a complete record of your achievements allows you to step back and assess your career trajectory. You can keep a list of career goals, with steps you need to take to reach these goals, in a private section of your portfolio and use it to track your progress and make realistic assessments of how you are doing in your career. You might also add in the private part of your portfolio a section on job satisfaction which tracks how much you did or did not enjoy different projects and work environments to help you think about career directions.