How does Shakespeare use the motif of sight in "We'll have thee, as our rarer monsters are/painted upon a pole, and underwrit/'here may you see the tyrant'"?

This quote can be found in the final scene of Macbeth, when the title character faces Macduff on the battlefield. Having previously believed that he could not be killed by anyone "of woman born," Macbeth is horrified to discover that Macduff fits this description, having been "untimely ripp'd" from his mother's womb. At this point, he curses The Witches, who he sees as having misled him, and says he will not fight Macduff. Macduff calls him a coward, which is what leads to the quote in the question. He says he will be put on public display (probably before his execution) in the same way that "monsters," or people with deformities, were. According to Macduff, he will be the "show and gaze o' the time."
Macduff plays on the motif of sight in an important way here. On the one hand, parading the defeated and humiliated Macbeth around will reveal him for what he is—a usurper who took the crown through murder. As a defeated tyrant, he will be made to suffer the scorn and the mockery, as well as the vengeance, of Scotland. Submitting to this fate would allow people to see something else about Macbeth as well—basically, that he is a coward. Macbeth has already responded to challenges to his bravery, most notably by his wife. Now he is goaded by Macduff into fighting, and dying, because he does not want to be seen as a deposed, illegitimate tyrant nor a coward. There is another sense in which this confrontation with Macduff, if not the actual quote in the question, involves the motif of sight. As he squares off against Macduff, Macbeth sees, too late, that The Witches have deceived him.