Another good question. If God knows every bad thing you do, why the need to confess to a priest?
Maybe the priest needs to hear a little something wicked now and then to get him in the mood?
My girlfriend is Catholic, so when I took her to the University of Illinois for a visit, she asked me to take her into the very large Catholic church on campus. It was the first time I ever stepped foot in the building. At the time, a priest was there taking confessions from college students. I thought that must be frustrating as hell living a celibate life, and hearing one attractive college student after another talking about their sexual exploits.