Why One Needs to Integrate RFID into WSN?

I'm not sure I completely understand the question... But...

A wireless sensor network can contain a variety of smart sensors who can communicate to each other, via a wireless network. These sensors only output a given type of data depending on what they are measuring/sensing (Flow/Pressure/Temp etc). The sensors do not contain any user programmable data, only give an output, and cannot be written to.

The RFID contains a unique programmable set of data, which can be anything the user wants to put in it. RFID are great for production lines, where an RFID chip is placed on the product, and as it travels through the line readers/writers can read which steps it has been through, and any data about them, and when the next step is complete, a write head can write new data about the newly completed process onto the chip for the next station to read etc etc etc. You can also program shipping/customer information into the chip when it's complete, and the shipping department can scan the chip, package the product, and send it off to its customer.