Peer assessment works best when there are very few "right" or "wrong" answers.

In law, responses are matters of opinion or interpretation, upon which it is possible to to disagree and still be "right".

Noble (2017) suggests (p446, full article in the resources section later) that in law teaching

Peer review is seen as an opportunity for students to comment upon, critique, and suggest amendments to other students' work, promoting interactive learning and developing the critical analytical skills of all the students involved in the activity.

This also promotes reflective learning in their own work, while enhancing the quality and the nature of feed-back dialogue when student numbers are large, without necessarily increasing demands on academic staff.

Provided that: (Sadler, 1989)

students have knowledge of the standard or goal to be achieved by the assessment
students assess work against that standard or goal of good performance
students later take action to improve their work to meet that standard or goal, and thereby close the gap between their present work and what is expected.