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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.

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With hard disks, I could see reading the compressed version from disk and decompressing giving you the bytes faster than reading the uncompressed version. With SSDs that advantage likely went away.
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I would guess to save on installed size?
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