Position descriptions can make structure rigid. They can slide into obsolescence and suck up time in unproductive tracking and fine-tuning. They might make it harder to fill positions or create new ones.
More positively, position descriptions help to clarify decision-making levels and employees’ roles. They pitch in during evaluations, promotions and succession planning, to the extent that the descriptions are up-to-date and people use the descriptions to figure out what they need to do to be promoted.
Bear in mind my libertarian views when I sense bureaucracy and administrative burdens. That said, position descriptions strike me as much more a reflexive need of HR than a useful tool that helps a general counsel find and nurture talent.