YES, you need–have to have a place to plug in your o2 sensor. The o2 sensor reads the air/fuel ratio and tells the ECU which adjustments to make to keep you car from running too much or too little fuel. The bung is just the fitting that allows the sensor to screw into the downpipe.
Not to be disrespectful but if you don’t know the relavance of having an o2 sensor, you really shouldn’t be attempting to boost anything just yet.
I would suggest to keep reading and learn all you can. There’s a guide here you can read if you haven’t already as well as numerous other sites that will help.
even if you shut the o2 sensor off (like you can in crome) you still need a spot for a wideband tune. Having the tailpipe wideband isn’t the most accurate.
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