The board is for the non-profit that ultimately owns and totally controls the for-profit company.
Everyone that works for or invests in the for-profit company has to sign an operating agreement that states the for-profit actually does not have any responsibility to generate profit and that it's primary duty is to fulfill the charter and mission of the non-profit.
To allow OpenAI to raise venture capital, which allows them to exchange equity for money (ie, distribute [future] rights to profit to shareholders)