问为什么在C＃中捕获后却重新抛出了异常？

public static string SerializeDTO(DTO dto) {

    try {

        XmlSerializer xmlSer = new XmlSerializer(dto.GetType());

        StringWriter sWriter = new StringWriter();

        xmlSer.Serialize(sWriter, dto);

        return sWriter.ToString();

    }

    catch(Exception ex) {

        throw ex;

    }

}

public static string SerializeDTO(DTO dto) {

    XmlSerializer xmlSer = new XmlSerializer(dto.GetType());

    StringWriter sWriter = new StringWriter();

    xmlSer.Serialize(sWriter, dto);

    return sWriter.ToString();

}

try {

    // Do stuff that might throw an exception

}

catch (Exception e) {

    throw e; // This destroys the strack trace information!

}

try {

    // Do stuff that might throw an exception

}

catch (SqlException e) {

    // Log it

    if (e.ErrorCode != NO_ROW_ERROR) { // filter out NoDataFound.

        // Do special cleanup, like maybe closing the "dirty" database connection.

        throw; // This preserves the stack trace

    }

}

catch (IOException e) {

    // Log it

    throw;

}

catch (Exception e) {

    // Log it

    throw new DAOException("Excrement occurred", e); // wrapped & chained exceptions (just like java).

}

finally {

    // Normal clean goes here (like closing open files).

}