Peer review is not only a process for peers to determine grades for one another. The value lies in the extra feedback that is received but also in giving feedback to others.
When students evaluate the work of their peers they memorise subject matter better and acquire new knowledge. They need to be able to understand the study material and reflect on it.
Having some ownership of the assessment process increases engagement with the subject. More engagement makes for more motivated students; a key success factor in increasing learning efficiency.