Why did Lady Catherine come to visit Elizabeth?

Lady Catherine tries to impose her high-class snobbery on Elizabeth.  She believes that Darcy may wish to marry Elizabeth, who is, in Lady Catherine's opinion, of a very low class.  She tries to tell Elizabeth that she may not marry Darcy because that would lower Darcy in everyone's opinion.  However, her real reason centers on the fact that she believes Darcy will marry her daughter.  Elizabeth does not give in to this kind of snobbery and Lady Catherine leaves full of frustration.