A bad system is worse than no system. Those to be evaluated are involved in creating the process. Adults going through a process of evaluation are powerful role models for students. Evaluation raises the level of professional discourse within the school. Good teaching is a reliable guide to good evaluation.
Evaluation makes everyone anxious.
Especially when you are getting started. Human nature resists the concept and practice of evaluation, but most people seem to value the experience. Peer evaluation is a crucial part of a good process. Good self-evaluation is even more crucial. Evaluation presupposes some benchmarks.