An imaginary conversation gives you complete control over someone whose real response is what you wanted.

The conversation you rehearse is one you cannot lose. You supply their lines, so the other person concedes at the right moment, and what feels like preparation is often a performance in which you have cast someone else as an agreeable version of themselves. Nothing is learned, because nothing could have surprised you.

Use the rehearsal for your own part only. Work out what you want to say and why it matters, then stop scripting theirs. Better still, go into the real conversation with a question rather than a case, because the actual person will say something you could not have written, and that is the whole value of talking to them.