I think Matthew Henry explains it quite well:
[i]God's friendly talk with Abraham, in which he makes known to him purpose concerning Sodom, and allows him a liberty of application to him about the matter. (1.) He tells him of the evidence there was against Sodom: The cry of Sodom is great, Gen_18:20. Note, Some sins, and the sins of some sinners, cry aloud to heaven for vengeance. The iniquity of Sodom was crying iniquity, that is, it was so very provoking that it even urged God to punish. (2.) The enquiry he would make upon this evidence: I will go down now and see, Gen_18:21. Not as if there were any thing concerning which God is in doubt, or in the dark; but he is pleased thus to express himself after the manner of men, [1.] To show the incontestable equity of all his judicial proceedings. Men are apt to suggest that his way is not equal; but let them know that his judgments are the result of an eternal counsel, and are never rash or sudden resolves. He never punishes upon report, or common fame, or the information of others, but upon his own certain and infallible knowledge. [2.] To give example to magistrates, and those in authority, with the utmost care and diligence to enquire into the merits of a cause, before they give judgment upon it. [3.] Perhaps the decree is here spoken of as not yet peremptory, that room and encouragement might be given to Abraham to make intercession for them. Thus God looked if there were any to intercede, Isa_59:16.[/i]
Krispy