Standards provide a focus for reform efforts -- all students must reach them. And teachers can see how well they are doing by looking at their progress towards standards.
Focus is one of the greatest benefits of standards; publication is another. Everyone can see what the schools are aiming to teach and what students must learn. What must be learned isn't a secret, kept for a small portion of the student population and hidden from the rest. Done well, standards can be an important tool for equity: if all kids are required to meet the standards, all schools must work to make children reach them, not just schools which have a majority of middle class, college-bound students.
Because standards provide a focus, they provide a yardstick for evaluating all aspects of schooling. Is this a good textbook? It is if it provides opportunities to meet the standards. Is this a worthwhile staff development workshop? It is if teachers learn techniques for getting students to standards. And so on. All resources, materials, schedules, personnel assignments, should be judged by this criterion: if we do this, will our students achieve the standards?