Usually you need non-free-firmware for using firmware-requiring hardware directly, in many cases wireless chips or GPUs. A rule of thumb is that inside a VM you usually don't need it, whereas if you're running on the metal without ensuring you run on compatible hardware there is a high chance you will need it. Also in case you discover later that you need it, you can always enable it. Not sure whether the dialog comes up on every installation or only if it detects hardware that needs it.

Yes, I thought I read somewhere that the dialog would only be brought up if firmware is found that requires it, but can neither verify nor deny based on my experience for now.