It's important for tuning. If you want to get custom tunes done, especially remote tuning via email, then you need to be able to log actual air:fuel ratio so that the tuner can compare the actual reading to the commanded value and figure out how to adjust the tune to get more perfect fuel mixtures.

You don't feed the signal to the ECU (can't be done; ECU has no input for wideband sensor). You feed the signal to a fuel ratio gauge if you want to monitor it as you drive, and also feed the signal to some device that you will be using to record data logs. For example, the Diablosport inTune supports up to 2 external analog signals that can be logged along with all the other stuff it can log.