Earlier quoted context omitted.
I think you misunderstood his question. Given two files, a compressed file which must be uncompressed to play, and an uncompressed one which does not require extra processing - why would the game ever bother uncompressing te compressed file when the same data is already available uncompressed? There's no true use case for that that I can see.
I would guess to save on installed size?
In this case it was stated that both the compressed and decompressed versions live side by side. So there is no install size saved.