Does your question mean, "What is the purpose of performance appraisal?"
As far as I can see, performance appraisal (PA) has two main purposes:
1. Reward Management, i.e., increments, promotions, bonuses, etc., and
2. Development Objectives, i.e., to identify weak areas and improvement opportunities.
Sometimes, one area is more dominant than the others. Take, for example, a forced ranking system in performance appraisals. People are placed into particular categories or forced, if you will. The focus in such a case is usually on the disbursement of increments/rewards, i.e., only 50% of the employees will get a 15% raise, the remaining others will get less, etc.
However, other systems that focus on development will look at the performance appraisals as a learning opportunity. Suppose the performance results of a department are plotted on a graph showing the number of average performers, weak performers, and good performers. Then the direction of the graph will indicate where you stand. If it leans towards the good side, then you know you are doing well. If it leans towards the weaker side, then you know there may be issues that need to be identified and addressed.
I have also heard of people implementing 360-degree appraisals solely for the purpose of feedback and growth (not for rewards) and running a separate system for rewards.
Hope that answers your question.
Regards