These files are purely for your enjoyment outside the game. If you delete them, the game's audio and gameplay will remain unaffected.
Soundtracks in lossless formats (like FLAC) or high-res artbooks can easily reach several gigabytes. Optional-Bonus-OSTs.rar
The full game soundtrack, often in high-quality MP3 or FLAC formats. These files are purely for your enjoyment outside the game
Because .rar is a compressed format, you will need a utility like 7-Zip or WinRAR to access the contents. Optional-Bonus-OSTs.rar