"Helpful in decision-making" is a curious way of speaking about mental health.

For clarity, I meant "learn enough to be very helpful to you in your decision-making about your condition", rather than useful to the doctor. (Which is not to say that it might not also be helpful to the doctor.)

For a lot of mental health issues it matters whether a patient feels agency.

Yes, absolutely - so for mental health issues it can be particularly helpful to learn enough about your situation and possible interventions so that you can be more involved in the decisions about them, rather than the locus of control lying with the clinician.