Isaiah 40 marks a time-shift in the book of Isaiah.
No longer are we dealing with messages to people facing the judgment of God via exile. Now the messages are for those people who've lived through the tumultuous Babylonian captivity.
God wants his suffering people to know that he hasn't been defeated, hasn't abandoned them, and hasn't forgotten his promises.
Isaiah 40 and 44 give us glimpses into God as the Saving King who offers us hope.