You're not wrong, but their value is that they do this for other companies as well and they have staff that have experience and competence in the subject matter

You're not wrong, but their value is that they do this for other companies as well and they have staff that have experience and competence in the subject matter. So, in their attempt to support the executive, they find out that they're heading in the wrong direction, they must advice them accordingly and help them come up with a strategy back-pedal. In other words, if they have new information, they will share that information to see if their clients will have a change of mind. If the new information (or lack of one) won't change minds, then they will submit their advise in support of what they were hired to do to begin with. These consulting firms, on their own, have had an enormous impact in shaping entire economies and countries. They are not mere props, supporting the ego of an executive. A lot of times, it is the entire board that hires them, or their report by the CEO is presented to the board, not one person. if the outcome is undesirable, they will lose credibility the next time they're used at that company.