John Calvin Commentary Isaiah 7:10

"And Jehovah spake again unto Ahaz, saying," — Isaiah 7:10 (ASV)

And Jehovah added to speak to Ahaz. As the Lord knew that King Ahaz was so wicked that he would not believe the promise, he commanded Isaiah to confirm him by adding a sign. For when God sees that his promises do not satisfy us, he makes additions to them suitable to our weakness, so that we not only hear him speak but also see his hand displayed, and thus are confirmed by an evident proof of the fact.

Here we should carefully observe the use of signs—that is, the reason why God performs miracles—namely, to confirm us in the belief of his word. For when we see his power, if we have any hesitation about what he says to us, our doubt is removed by seeing the thing itself; for miracles added to the word are seals.