I still see engineers storing user-uploaded photos in /var/www/uploads/ on an ext4 volume and wondering why their server falls over at 10M files. Meanwhile, the team next door threw the same photos into an S3 bucket and scaled to 100M files without breaking a sweat. The difference is the storage paradigm. Pick the wrong one and you feel it at scale.
I still mix the two up in conversation sometimes, so I keep a short checklist: random writes and file locks mean file storage; HTTP PUTs and billions of objects mean object storage.
Short answer: use file storage when you need POSIX semantics — in-place edits, sub-millisecond random I/O, file locking (databases, OS files, NFS shares). Use object storage when you need scale, an HTTP API and rich metadata (user uploads, data lakes, backups, ML datasets). Most mature stacks run both, side by side.
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