You can duplicate the functionality of computer games (and other computer programs) without decompiling them, in some cases. However, this can require a lot of testing to figure out the working. Having a reimplementation (whether due to decompilation or due to figuring out the behaviour without decompiling it) can be helpful to explain the game rules better too, which many computer games don't explain very well in their documentation, so this means it is possible to write a better documentation (one example where this is helpful is Pokemon). (I have figured out the working of Hero Hearts without decompilation, and making it (at least in my opinion) better than the original, without needing any LLM/generative-AI to do this. Some people (including myself but mostly others) have guessed at (and tested) some behaviours of ZZT, both by testing and by partial decompilation, but eventually someone did rewrite the source code which produces the same executable file than the original, so it is known that it is the correct implementation.)