@carlsbad is right, the extra band is a dedicated backhaul between the satellite access point and the main access point which is connected to your ISP. Having this dedicated channel increases throughput of the devices connected to the satellite since the satellite doesn’t have to contend with devices connected on the main access point. These days with 802.11ac and 802.11ax, speeds are fast enough for any hi-res file you’d want to play, whether connected through wi-fi or Ethernet.
I have an ASUS XT8 tri-band mesh LAN. The streamer (a Raspberry Pi running moOde) is hardwired to the main access point. The music library sits in a shared folder on a Mac mini connected via wi-fi to the satellite. I don’t have a problem with streaming hi-res Qobuz from my phone through the RPi using UPnP / wi-fi or listening to my music library. The sound I get is equivalent to disc-based playback through my Oppo UDP-203 with VanityHD digital output board.
I chose the XT8 802.11ax system because my internet connection is ~1 Gbps and wanted max throughput. Using 802.11ac, I usually get speeds around 580 Mbps over wi-fi. Note that even for uncompressed PCM 24 bit / 768 kHz sampling rate, the max bandwidth you’d need is ~37 Mbps. Even Fast Ethernet can handle that.
The only adverse effects you’d notice if you didn’t have enough network bandwidth or bit errors during transmission would be dropouts and glitches / pops during playback. Otherwise, the data is getting from point A to B intact.