And when you want to share some but not all files with that one user but not other users you created for similar purposes?

And when you want the outputs of that user back to your main user?

And when you want that user to access some shared credentials for external services, but not all?

It’s not the account setup that’s hard, it’s the workflow of spreading a single real-world across multiple accounts.

All of those use cases are very easy to facilitate using filesystem permissions and groups.

That’s what user groups are for.