When we began discussing the BrewHow app, we went through a rather lengthy description of both beer and the brewing process. Though I admittedly love to talk about beer to anyone willing to listen, and occasionally those who aren't, I presented the information to help you understand the concepts relevant to beer and its production. Why is knowing how to make good beer important to writing the BrewHow app?
I will answer the question with another question. If someone were to ask you to write an app to determine the air-speed velocity of an unladen swallow, could you do it? Were you to go it alone, you would need more than software development skills. You would need a good understanding of aerodynamics and kinematics. Given that's not likely the case—and kudos to you, dear reader, if it is—you would need to consult with a domain expert.
This same principle applies to designing software; not just software about beer or swallows (African, European...