Why scans with no symptoms?

I am her2neu positive, and my doctor believes in no scans, and I am really really happy about that. Scans freak me out, and frankly, I don't want to start treatment for something sooner than I need to. If it doesn't matter WHEN you start treatment for mets, then why does it matter whether you wait for symptoms before ordering a scan?

What are the benefits of early detection of mets? I thought that once it's "out of the box", it's all equal in terms of treatment and prognosis...