To understand the capabilities in modern terms, imagine if each file was a SQLite database in addition to containing its normal data. The normal data and the database would be entirely independent "chunks" of data. The system APIs would offer the ability to either open the file as a normal data stream, or to query it as a SQLite database.

In such a world, you might find yourself leveraging the database "side" of executables to store assets for your programs. You might use it to store the fonts used in your documents. And you might use it to store metadata for your photos.

In some cases, you might find yourself creating files just as an easy way to get a SQLite database, and not putting any normal data in them.

The format wasn't actually SQLite, but in a hand-wavey way, that's resource forks.