We all know, at least on Desktop, the trusty Preview Window to the right as we type.
You know it is trusty. I know it is trusty. But User C does not. Therefore to increase trust, the Preview Window should be fitted with a Refresh button. That way if the user is ever worried that what s/he sees might not be what s/he will get, they can click it.
Yes, the Preview Window is supposed to refresh automatically. So in theory there should be no need for a Refresh button.
However just like all browsers also have Refresh buttons, as things don’t always work out as planned, it’s better safe than sorry.