Why do we pray for the dead? Because of the Resurrection. Jesus explains this very clearly in His answer to the Jews who asked him about the woman who had 7 husbands (Luke 21:12-19). As we talked about this, the best two answers to the question "What is different between Orthodox and (fill in your Christian denomination here) came out. They involve the Resurrection and mercy.
We also talked at length about what the purpose of liturgy is and the first three secret prayers the priest prays. He prays with more intensity, since he is in front of the altar, but we all must learn how to pray as these prayers. These prayers answer the common question I get: "What prayer can I pray for (fill in the blank).