When doing problem discovery interviews, this is such a valuable question to ask. And it's counterintuitive because we aren't supposed to ask the user to design the solution. After all, they are only experts of their problems but not the solutions.
I like this question because it reveals what constraints the user operates under.
- Researcher: "How would your ideal solution look like?"
- User: "...hm, not sure. You could put an export button here. It would start the export, but then I wouldn't have to wait it out. Our database is huge... would take an hour. I have seen other products email the exports once they are ready, but that wouldn't work for us because you can't attach that big of a file to an email, and I wouldn't want that for security reasons either..."
The design idea (a button at a certain location with a certain label) is not that interesting. But as users are trying to come up with the ideal solution, they are naturally considering their constraints, e.g. their database size and their high standards of data security.
I think of it as reverse brainstorming. In brainstorming, participants are specifically asked to put their inner censor on hold. Not to weed out any bad ideas. In a problem interview, all I am interested in is their inner censor. What's their inner censor saying? Why wouldn't certain ideas work? What constraints do they need to operate under?