Except I am not comparing anything to communism, but summarizing Stallman's manifesto as an appeal to communism, which it is.

I asked why someone would consider it necessary for such software to be free because I thought the argument would be about some functionality that already exists as free software or something that was taken from free software.

Yet OP just copy pasted an argument that is incompatible outside of an utopic setting, from a person that no longer has a place in modern society due to his wrongdoings.