Some applications, such as professional schools give applicants the choice to waive their right to view their letters or not. Usually, applicants are encouraged to waive their rights because if they do not, it is a sign they are not confident in their recommenders.
The most effective letters of recommendation are written by professors or work supervisors who know you well enough to describe your academic, personal, or professional achievements and potential with candor, detail, and objectivity. Letters that compare you to your academic peers are often the most useful. Most schools do not consider general, unreservedly praiseworthy letters helpful.