You need it if your agent can access the internet and read files you don’t want public. That’s a relatively minimal case.

Do you actually need it, or do you just fear you need it?

What's the actual realistic threat model for median developer or median user here?

By realistic, I mean that leaking your grandma's recipes or your SSN or your million dollar idea to some pastebin is neither likely nor going to meaningfully make things worse for you, or be useful for any malicious actor. Surely this is not what everyone is worried about?

It is really easy to restrict and contain an AI agent as long as you don't give it access to a terminal. If you only give it tools to read files and access the internet, then it is much easier to just restrict the tools themselves rather than setting up a whole isolated sandbox.