At the end of the performance review, you and your manager will sign a form documenting the results of the meeting and citing changes and plans for the future. Your manager will also collect your self-appraisal to submit to the human resources department with the other paperwork for your employee records.
Employers keep records of performance reviews for their protection in case of contested terminations, failure to promote, and contested training opportunities that could arise in the future. Your employee file protects you as well since it can prevent unfair assessments and ratings in future performance reviews. It is difficult for an employer to fire an employee for cause if there is nothing in the employee's permanent record to support such a decision.
In addition to legal reasons, documenting performance evaluations is important because it puts in writing the expectations your employer has of you. The agreement about your performance that you and your supervisor sign has three major functions: to communicate the work to be done, to outline the results to be attained, and to detail the skills and competencies needed to achieve those results.
You and your manager can refer back to these forms periodically as a reference on objectives and plans in future reviews.