Hello, I think that such setting would be useful if WiFi network is operating on both 2.4 GHz and 5 GHz bands, but your phone connects via 2.4 GHz band, while you want it to connect to access point via 5 GHz band. This might be useful in case if 2.4 GHz band is polluted by other WiFi networks, but phone still connects via 2.4 GHz.
