There are two audiences for this passage.
First, Paine is writing it for people who are undecided about which side (if any) to take in the Revolutionary War. He is trying to persuade them to come to the side of the rebels just as he did after the Battles of Lexington and Concord. He is doing so by comparing King George III to Pharaoh, implying that he is oppressing a people and refusing to let them out of slavery. He is saying that the King does not act as a true ruler should. In doing so, Paine hopes to justify the rebellion and persuade others to join it.
Second, Paine is writing to stiffen the resolve of those who are on the rebels' side. He wants to remind them of what he sees as King George's unjust actions. By doing so, he wants to ensure that they remain committed to the rebellion.