Most Civilization 3 Files when produced are compressed, which is problematic when doing things like hex-editing. The only exceptions are autosave .SAV files, which can be read without needing to be unpacked. The game and the Civ3Editor can both read decompressed files fine, so there's no need to recompress them after editing like is necessary for Civ 6.
This utility uses a JavaScript implementation of a script originally written in Java by user Quintillus on the CivFanatics forums. See this thread for the details. Point being, full credit goes to Quintillus for the code. I've simply adapted it into an online portal.