Before technology became prevalent, writing letter was a primary form of communication. The brevity of the method caused words and emotions to be more thoughtful and more valuable.  During Biblical times, a letter of commendation was often sent with a person to authenticate who they were, and the apostle Paul used that as a parallel for our lives as believers, as "letters of Christ". This causes us to evaluate how well we represent Christ and communicate our relationship with Him.  Who people know us to be, how we show them our convictions and our purpose for living, and what we call them to determines how effectively we live out our faith.