What are introduction letters?

Introduction letters are letters the ladies compose and have their agencies send for them. The lady composes the letter and tells the agency what type of men she would like to correspond with and then the agency sends the introduction letter to those men. Why would the ladies prefer to have the agency send an introduction letter for them? The women are busy with school/work, etc. They are not going to spend hours on end sending 100-500 letters in hopes that one or two guys respond to her. Therefore they write their introduction letters and give their agency guidelines on the type of men to send the letter to. When the man responds they start the correspondence if she is interested in him. The women can come into the office and send their letters themselves, but few do because having the agencies do it is much easier and convenient for them.