Imprecatory Psalms

The Psalms are filled with language of judgement against God's enemies.  The end of Psalm 83 says, "Let them be put to shame and dismayed forever; let them perish in disgrace."  

What is the purpose of these prayers?  Psalm 83:18 gives us the ultimate answer:

that they may know that you alone, whose name is the Lord, are the Most High over all the earth.