It is required if a game does not calculate correct viewports compatible with WFOV/ canted screens. As a result, both views are geometrically displaced, shadows / light are incorrect, massive object culling, etc.
However, Pimax enables use via parallel projection, where compatible images are created from the rendered data.
However, around 30% additional GPU performance is required for this. (simple explanation)
You can read soe things about it here:
Pimax Parallelprojektion - Pimax 8K Serie - OpenMR | Gemeinschaft
For standard 100degree HMDs it doesn’t matter because the screens are parallel to your eye and the field of view is just much, much smaller.
The index also uses PP, but to a lesser extent and automatically in the background. XTAL identical to Pimax.
StarVR users are unfortunately unlucky.