So it is a critical document that mostly used in financial life of American citizen, so it could be used in various cheats in financial sphere. Am I right?

Does that clear it up :) No? Yeah well welcome to America.

There are tons of weird things of that type in every country you could find. F.e. we have "pension insurance number", "individual taxpaying number", "mandatory medical unsurance number" and it's not easy to explain to a foreigner what they are in real life and how they really used.