There are situations where a contact has a specific title that should appear on letters (mail merge) that is different from the current Prefix field.
Example: If the Prefix field contains a contact's title such as 'The Most Reverend' then a letter salutation would turn out to be: 'Dear The Most Reverend Salvatore R. Matano' which would be awkward.
The salutation should really be 'Dear Bishop Matano'. In this case the Prefix would be 'Bishop'