The complete bike only makes up for 25% of the overall drag, the wheels even less at 8%, while the rider causes 75% of it. So why is the wheel still important in this calculation? In order to stay aero in the first place, you have to stay comfy no matter the riding situation, because only then will you be able to keep an aero position, which affects those big 75%. The wheelsets have a huge influence on handling characteristics and this is why wheels are key when it comes to optimization of the handling.
The steering moment describes the force the rider needs to apply to the handlebars to keep the front wheel straight. Over the relevant yaw angles the change in steering moment should be as even as possible and show as little spikes as possible. If these demands are met, the wheel has good handling characteristics and allows you, the rider, to stay aero.