Why does God say in Genesis 18 that He will go down to see if Sodom is as bad as the outcry?

This is true, so we do not explain it away as unreal, as many exegetes do who have no real respect for Scripture; it is inspired by the Holy Spirit, who cannot lie, so it is the exact historical truth. Why then would God speak this way? Because in Scripture even God has to come under his own laws. One of God's own laws is that you cannot make an accusation as a single person and have it stick in a court of law; that is why one of the first laws in the Pentateuch is that every matter is established in the mouth of two or three witnesses. When you testify, every single word must be the truth, and God binds himself to that same standard, because the whole definition of who God is, is one who cannot lie. So when God is dealing with Sodom, he speaks as one who investigates and confirms, going down to see, so that the judgment rests on established testimony rather than a bare accusation. He is, in effect, being a witness to himself, and with the Holy Spirit or the Son you have the second and third witness, so the matter becomes legally adjudicable. It is not that God lacks knowledge; it is that he submits his own dealings to the legal standard of witnesses that he laid down, which is the same reason that in Hebrews 6 he doubles his oath to Abraham, so that by two unchangeable things it is absolutely certain.