Why do Roman Catholics show devotion to the Sacred Heart and Immaculate Heart? I wonder why Roman Catholics show devotion to the Sacred Heart of Jesus and Immaculate Heart of Mary if their devotion can be shown to Jesus Christ and Mary instead? What's the difference?
Ok.  I don't know if this will be an appropriate answer, because I have not had the time to dig deeply into finding any expert Catholic writings on this specifically.  Pope Pius XII's Encyclical HAURIETIS AQUAS on devotion to the Sacred Heart is about the closest I can find in the near term, but I have not read it fully to see if it addresses your question sufficiently.

By my own reason though, I think of the analogy to understanding a very complex subject, design, etc. in our earthly life or to understanding any given person.  If you contemplate a very complex thing as a whole, as humans, it is hard to understand it at a very deep level.  If we narrow our focus of the subject in our contemplation, then we start to develop a deeper understanding of the larger subject.  I believe that is a large reason that the Catholic Church recognizes various titles, roles, and view points on Jesus, Mary, God the Father, the Church, and even ourselves.  It is to allow our limited human perception to penetrate the hugely complex (infinitely complex in the case of God) to understand them much more fully.