He didn't need to go down see whether they sinned. He already knew they had. But he wanted to see if there were any righteous people left in the city who could save it from destruction. The Bible says that God knows everything, so he would have known exactly how bad Sodom and Gomorrah were. But he wanted to give the people a chance to repent, and maybe there were some good people left in the city who could have turned things around. Unfortunately, there weren't any righteous people left, and Sodom and Gomorrah were destroyed.