When you go to see the doctor and he proposes a procedure which is not without risks, it is normal that you ask for a second opinion. The same is true for engineering projects. The higher the risks, the more worthwhile an independent review can be. I.e. when your department is preparing a project which is exceptional complex, large or expensive.

Some of the aspects on which a review focusses are:

Completeness: is the description complete or does it leave tasks out which may lead to obstacles and/or extra costs?

Correctness: is the description realistic; are the prerequired conditions sufficient; are the estimated number of hours enough?

Testability: are the requirements clear enough to be tested? Which requirements could use more detail?

Necessity: are the costs worth the benifits? Do third party solutions exist which could do the job?