Repacks are designed to save you bandwidth and disk space. By making the soundtrack a separate .bin file, the repacker allows you to:

In conclusion, fg-optional-bonus-soundtracks.bin is more than a stray filename. It is a mirror held up to digital culture: we hoard optional bonuses, we legacy-mire soundtracks in opaque formats, we shrink entire albums into .bin containers, and we call them “optional” to avoid admitting that what is optional is often first forgotten. Perhaps the most honest act of digital archaeology is not to open the file, but to read its name as a small epic—where “fg” stands for forgotten gem, and the only track that plays is the hum of curiosity.

By keeping it optional, the main game download size is reduced. For example, in The Sims 4 repack, skipping this file can save roughly 344 MB of data traffic.