Why is the study of letters important for our understanding of the Bible?
Because over 30 percent of the writing in the New Testament is contained in letters. They are: Romans, 1 Corinthians, 2 Corinthians, Galatians, Ephesians, Philippians, Colossians, 1 Thessalonians, 2 Thessalonians, 1 Timothy, 2 Timothy, Titus, Philemon, Hebrews, James, 1 Peter, 2 Peter, 1 John, 2 John, 3 John, Jude
Why do some call these epistles and not letters?
Deissmann insists (p224) that most of the letters in the New Testament are letters and not epistles.