We live in an age so devoid of sacrifice that it's hard for us to wrap our heads around why this was so common and so important. This act would add a layer of irrefutable legitimacy to the groups religious beliefs, and as such would make those beliefs into a unbreakable social bond. Grief is a powerful social glue, and when you use it so intentionally and solemnly it becomes a core part of the social fabric that held these groups together for hundreds of years in an ultra-hostile environment.