A subpoena duces tecum is a subpoena whereby the person subject to the subpoena is ordered to bring and produce certain documents.  Therefore, if you have been served a subpoean duces tecum then that document should indicate what documents you are ordered to bring to court.  Thus, the first step in determining the purpose of the subpoena duces tecum is determining what documents the prosecutor is seeking to get.  Once you determine that, it might be possible to infer why he/she might want to look at them.  In any event, issues like these that arise are one of many reasons why it is a good idea to have a criminal defense attorney representing you in any criminal matter.