To help a client choose what to do.
A consultant is only a consultant when they help the client decide what to do. The client has the power to make a decision; the consultant does not. Or rather, if the consultation ends with the “consultant” choosing on behalf of the “client”, then consultation has not taken place.
The client has agency and struggles to express it. The consultant helps them express the agency that they have. That is consulting.
(Plenty of healthy, desirable relationships are not consultative — your life partner does not become your client when they ask where you should eat dinner.)