Thanks for the reply.
So ok, I don't need the hand wand/controllers. What about those cubes that you place around a room to track your body motion? If I'm just sitting in a chair and turning/tilting my head around, do I need those?
Yes.
There are two types of tracking for VR headsets, so called inside-out tracking and lighthouse based tracking. The Valve index uses the latter; basically small boxes that emit coded light rays that allow the headset to determine its position. Without the lighthouses it cannot determine the headset's position and orientation in 3D space.
Other headsets like the Meta ones or the Reverb G2 I have use only camera and sensor based tracking; basically they scan your surroundings to get a position fix and additionally use a gyroscppe for rotation stuff. With those headsets, you don't need anything else, in fact you cannot use lighthouses at all with them.
So short answer: The Valve needs the lighthouses. If I am not mistaken you can get away with two of them for seated VR, if you do big roomscale stuff you might need three. But I am no expert there, I've always only had headsets that do inside-out tracking.