How many times, in a conversation with a client or prospect, you've heard their problem was X only to find out in the middle of the engagement that what they really needed was Y?
Chances are their self-diagnosis is wrong, or at least biased and incomplete.
That's why every consultant has a professional responsibility to ask whether the problem a client presents is what they most need solving. You need to diagnose before prescribing. Overlooking this is harmful both for you and your clients.
I was talking about this topic with a consulting partner of a firm in the IT space, and was surprised to hear from him that this was a "contrarian take". When did putting your clients first and following a fiduciary standard become contrarian?
Very often, the best way you can add value to your clients is by helping them define the real issue. As one of my favorite quotes says:
More value is added through problem definition than problem answer.
Indeed, some friends argue that executives who can accurately determine the roots of their troubles do not need management consultants at all.