It has the ability to read AFR into a range that the narrowband cannot. In otherwords, say you are trying to tune for 11.0 to 11.5 AFR at WOT, the wideband will read that rich, the narrowband cannot. It will read stoichiometric and a small amount in each direction, both leaner and richer, but not much beyond that.
I don't think the narrowband works at all during WOT. I think the fueling works off an algorithm at WOT that using things like MAP, RPM, TPS, and other sensors to determine where the fueling needs to be. But, I am more of an OBDII guy, so this has been a learning curve for me as well. Maybe one of the old timers can chime in.