There are selectable and simultaneous dual-band devices. The first type lets you switch between the 2.4 GHz and 5 GHz bands, while the latter allows using both bands at a time, which becomes handy for devices like NAS, media streaming servers, gaming laptops, etc.
Devices that require higher connection speed will benefit from using the 5GHz band, while other devices, like smartphones, tablets, and regular laptops can use the 2.4 GHz band and benefit from a broader operating range.