If I may throw in my 1/50 of a $, the IG method is supposed to appear to the connected equipment as a star-ground system, where every EGC is terminated at the same, well, terminal, and not depend on a web of EGC's and/or conduits.
The idea is to eliminate possible voltage-gradient-caused potential differences between the chassis of individual electronic devices. This was more important before UTP cabling became the norm, and shielded data cabling was popular.
We still see the effects of such problems (ground loops), when we receive posts about video hum bars and audible hum in AV and audio systems, respectively, where different circuits supply physically-separate electronic components.
We've certainly read enough threads discussing the need for EGC's run within conduit that we know conduit systems are less than 100% reliable for low-impedance nonding paths, and high fgrequencies are affected differently than 60Hz.
Is the IG system necessary today? Much less than in the past. By the way, if you're using NM and plastic boxes, you already have IG wiring, except for home-running the insulated EGC to the main grounding bus when you have sub-panels.
