If, say, you design something and then you ask people, "Why do you think this is good or bad?" Or if you're a journalist asking a politician, "So, why did you make this decision?" Or if indeed you are a politician and try to explain why a certain decision was made.
Si diseñas algo y luego preguntas, "¿Por qué piensa Ud. que esto es bueno o malo?". O si eres un periodista que entrevista a un político, "¿Por qué tomó Ud. esta decisión?".