Having the option to hide the marking period cumulative grade from students focuses the student on the current learning and not a number.  

Logistically, if a teacher sets percentages for assignment types (for example...quizzes = 25%, tests = 50%, projects = 25%) and then for whatever reason those percentages change, a student's grade could greatly be impacted/altered.  Which could then cause undo tension with both the student and their parents.