Agreed, most northbridges now don't have fans, although with a water cooling setup one is usually added since there is no heatsink fan to cool the motherboard parts. So that's why gigabyte provided a NB fan header.
Its not neccessary to add a NB fan, or to get a 3pin fan for your exhaust but speed monitoring is a nice feature.
Likewise the sysfan is a 4 pin to allow a PWM southbridge fan that is speed controlled by the bios similar to CPU fans. It should also take a 3 pin fan as well.