Arbitrary time boxes are what poor managers use to micro manage. I recently saw a team put under leadership of a poor manager and descend into never ending micro management under the guise of "milestones" which were just arbitrary time boxes determined by a tech lead. It also serves to pressure the tech lead "you said it would be done!" which thereby incentivizes the tech leads to pressure their team. It's really dumb, net loss for the company (burnout, higher turnover), and a total waste of time.