When the game was originally installed, main.pack contained the main files. It still does.

The way CA patch E:TW means that they add changed files to patch.pack

This means basically the original files are still in main.pack, but when you load the game, any files in a .pack folder that is labeled a 'patch'.pack (not just patch.pack) overwrite anything in an original.

In the same way, any conflicting files in a .pack labeled as a 'mod'.pack overwrite any in a patch.pack which causes a lot of mods to stop working after a patch, as new files are needed, and they arent in the mod pack.