Well, regarding God's listening to and answering Prayers. Indeed, the most part of humanity has the valid intuition that while there is a Providential God who listens to and commiserates with some human beings, it is clear that God does not listen to them all.
The Catholic Church as known and recognized this fact since almost forever, and so believers are categorized as either ordinary, or somehow 'blessed'. Saints are defined as those whom God chooses to deal with.
Now, in the instance of people sending letters to some Preacher, well, then, we must assume that they regard him as something of a Saint. Knowing that God is not listening to their own prayers, they ask some Preacher or other Holy Man to 'intercede' for them. The Theory is correct. Only they failed to discern a True Saint.
Saints are rare. Right now at this moment I could not name a Catholic Saint.. not a living one. Oh, there was Father Nacho who was raising the dead to give them their last rites. You see, Last Rites are supposed to be given while the people are still alive, but everyone is careless and they call the Priest too late, and when he arrives the patients are already dead. Well, most Priests play along and simply give the Last Rites to the dead body, and cross their fingers about it. But Father Nacho would cross them a few times and they would 'revive' long enough for the Last Rites to be administered, and then they would die again. Well, the Doctors and the Nurses were extremely disturbed over all of this and so they asked Father Nacho to please leave the dead people dead. And so he agreed. But, certainly, we must regard Father Nacho to have some real connection to Providence. However, he is a frumpy rude little Priest and doesn't conform to the popular conception of what a Saint should appear like, and so nobody would think of him as a Saint.
The Other Higher Religions may also have Saints. There was a recent Marian Apparition in Medjugorje Hersegovina. some of the Seers there asked Her if there were any "Saints" in the villiage. Well, She was honest with them and told them that Saints were very few anywhere, but in that villiage, the closest person to the criteria was a certain little old lady. It turnes out that this little old Lady was a Muslim. Well the Seers were shocked. But Mary told them that who and who is not a Saint is entirely up to God.
There have been documented Saints in Sufism (traceable to Zoroastrianism although they claim today to be Muslim, probably so they don't get their heads cut off by their kind Muslim neighbors), Saints of Yoga (I don't call it Hinduism, because much of Hinduism is, well, racist or crazy), Saints of Mahayana Buddhism (to be distinguished from original Buddhism.. the philosophy of a spoiled child who wished to replace supernatural religion with a secular philosophy).
Also, where we have Intercession with God and His Angels, we have, on the other side, demonic intercession. So, in the Carribean and in Africa and in various places in the Latin World we can discover Sub-Religions that specialize in Curses, Spells and Charms. People are looking for help, and if God will not oblige, then they consult with the devils.
