You can log in every time. A user on a different browser, device, or domain cannot. Cookie SameSite settings, a redirect URI that still points at a preview host, or a session that only sticks on localhost will pass your own tests and fail in production.
FlowWatch signs in the way a real visitor does — open the app, hit login, complete the flow — so a broken auth redirect or a cookie that never lands shows up the same day, not after support tickets pile up.
If returning customers cannot get back in, revenue stalls quietly. Catch it before they bounce.