its for mounting the second o2 sensor further down the exhaust system. the stock protege second sensor is located in the j-pipe. since the first cat on the msp was behind the j-pipe, mazda used this extension to move the second sensor further back into the mid-pipe. you only need this part if you install a header on your protege and want to move the second o2 sensor to the midpipe, or if your stock msp harness extension is busted.