If God is omniscient why did he need to come down to earth to see for himself?

Genesis 18:21 (KJV) states: > I will go down now, **and see whether** they have done altogether according to the cry of it, which is come unto me; and **if** not, *I will know*. If God is all knowing, why did he have to come down to earth to see if what he had heard about the evilness going on in Sodom was true?