We're often not experts in the domain we're working on, but by asking questions about a user's workflow on a more holistic level, we probably have more insight into what workflow would be the most productive from a systems standpoint.
Of course, this is a back-and-forth. Sometimes users aren't sure of what they want. And sometimes we're not sure of what is the "best" system for what the users want.
Instead of asking the user where the button should be, ask them when they press the button.