The 2.4 GHz band (standard network) on your router is your go-to network. The standard network works best for devices that will be used outside of the room containing the router. It combines both 2.4 GHz and 5 GHz to support all devices with strong signal throughout your home.
In some cases, you may wish to use the 5 GHz dedicated band (denoted by a _5 after the network name, i.e. YourNetworkName_5). This band is great for 5 GHz-capable devices, and can be useful for streaming. Its range is not as extensive, so it should be reserved for devices in close proximity to your router.