Security: you don't need a NAS because your files don't need to be accessible "online" -- they're already local to your own computer.

Reliability: with local storage, you have the ability to make backups without re-downloading multiple terabytes of data each time.

And there's definitely no way your files could just spontaneously disappear from cloud storage, either:

https://www.forbes.com/sites/jaymcgregor/2023/11/29/google-i...