@Dominions Son
They are neither necessary for nor primarily used for the purpose of building a home buried in a natural or artificial hill of earth.
There are multiple different methods to build subsurface. ICFs are included in that. The difference with ICFs and a normal concrete wall subsurface build is the foam. Which btw, can and is incorporated into water proofing the structure via various methods. Application of bitumen on the outside surface for instance.
One of the issues inherent with subsurface builds is temperatures. While it is good for cooling, subsurface builds are inherently difficult to keep warm. In my area it is ~13.5ยฐC in the winter at 3 meters down. That varies by area, especially in areas subject to permafrost or desert conditions, but in most cases, below surface builds can be considered chilly to the average person.
Insulating such a structure after the fact can be problematic and expensive. Standard structural concrete has an R value between .08 and .325. Not very efficient at all. AirCrete is another option, but not for structural values. It's very often used for the aforementioned insulation retrofit.
I would suggest further research on your part before commenting further.