The standard design process asks what users need. That's a fine question, but it's not the one I spend most of my time on. The behavioural economics question asks something narrower and, I'd argue, more useful: given how people's cognition actually works, with its shortcuts and biases and context-dependence, what is this specific interface going to make them do?
Years of conversion rate optimisation across banking, insurance, loyalty, and e-commerce taught me that the answer to that question usually comes down to details nobody thinks to argue about: the order information appears on a page, whether a choice is framed as a gain or a loss, whether there's a default option at all. Get those wrong and no amount of good intentions in the brief saves the design.
The point is to understand how decisions are actually made, under time pressure, with incomplete information, shaped by context, and designing for that reality rather than an idealised version of it.