(1) You're usually better at it than they are, because they're hiring a consultant in the first place. (2) You're outside of their political network, and can thus be a more neutral, objective arbiter. Invaluable! (3) You're looking at the problem with fresh eyes, whereas they're looking at it with a history of counterparty relationships, failed or successful previous projects, etc.

Furthermore, requirements gathering and obtaining project design consensus is going to happen at some point. Better you verify that it happens as fully as possible, as early as possible, versus trusting the client that they've done it accurately and sufficiently.