The Sign Was Prophesied

Ephraim and Syria were threatening Judah. The continuation of the nation was at stake. It's permanent demise would mean the promises of God were null and void. Ahaz, king of Judah, was offered a sign to show that God would preserve Judah. He refused a sign from God. So God gave a sign to the house of David, the entire nation, that the enemies would be defeated.

The sign was the well known prophecy of the birth of Christ by a virgin. The fulfillment of this prophecy (Isaiah 7:14) was His birth (Matt.1:23). The birth of the Messiah taking on the form of man would be assurance enough that Judah would not perish until all was fulfilled.

By the way, the word "almah" translated "virgin" in the English and "parthenos" in the Greek Septuagint was used of a marriageable, unmarried, chaste woman; a virgin. Both Testaments affirm Jesus was born of the virgin and her name was Mary, skeptics notwithstanding.