The Arctis 5 does need the USB adapter to fully utilize the surround sound and microphone’s capabilities. This comes in part from the implementation of the SteelSeries software for controlling the headphone profiles.
The microphone isn’t usable with PCs with the native 3.5mm port, as you will need an adapter of some stripe to get audio input. Thankfully, the USB connector does allow for this and a lot more functionality on the headset.