The keys.txt file is a plain text file utilized by Cemu to decrypt Wii U game data. Think of it as a master key ring for your digital library. While smaller game files (like Loadiine format) might not require these keys, any raw disk dump (WUD or WUX) will not work without them.
recommends the following legal methods to dump keys from your own console: : Use homebrew tools like wii u keystxt for cemu