A AFR gauge is to measure the stoichiometric ratio of air to fuel. The theoretically optimal air fuel ratio is 14.7 pounds of air for every pound of fuel. At this ratio, theoretically, all available oxygen in the air combines with all available fuel.
A narrow band will only measure a narrow band or range of this ratio, typically 14 to 15 and only use approximately a 0-1 Volt signal, so you do not have that much range.
A wide band has a wider band or range. Typically it will measure stoichiometric ratios from around 7.5 to 22.5, thus wider band. They get this added range from a 0 to 5 volt signal, that is why they need a different O2 sensor and you need to add the extra ****.
For tuning cams and such, most tuners prefer and sometimes need the added range of the wideband to make the correct changes and to see what is going on with your tune to get you running optimally.
I hope this helps and makes sense.
