I spent some time doing employee evaluations today. I do not enjoy doing evaluations as they tend to worry employees if the don't get the highest rating. The thing is, you can't give the highest rating all the time. Otherwise, when they do an exceptional job, there is no better rating. And, if you give a low rating in some aspect, they take offense.
Still, evaluations are necessary. It is necessary for bosses to monitor how their employess are doing with respect to the items being evaluated. Evaluations help employees improve their performance because they are aware where they are lacking. There is the old saying that if you don't measure, you don't control. Thus, evaluations give employees a measurement they can use to control the outcome of their work.
Evaluations can be annually or quarterly, depending on the organization. Quarterly evaluations are better. This creates a tighter feedback loop in which the boss can inform the employee where they have improved or lost ground. It loses some effectiveness if evaluations are too far apart.