Problem of Z-Fail algorithm

Z-fail algorithm has the problem that the far clip plane clipping shadow volumes .Setting the far clip plane far in the infinite distance to avoid shadow volume intersections and capping the shadow volume is a robust Technique for Shadow Volume.
However ,z-fail algorithm has no problem with near clip plane. i do not understand why just setting the far clip plane far in the infinite distance will be ok? there are also other four planes of the perspective volume ,the shadow volume can also intersect with these four planes except near and far clip planes,why we just move the far clip planes?

Because you only care about what you can see, and you can only see what’s between the Left, Right, Bottom, and Top clipping planes. There are no stencil pixels/samples for areas outside of that region.

BTW, besides pushing the far clip out to infinity or generating far caps for your shadow volumes, another solution to Z-fail’s far clipping problem is to use ARB_depth_clamp.

Thanks a lot !