Apart from the fact of meeting very nice and inspiring people on Couchsurfing, I find it really helpful to know somebody how knows where to eat the best food, have the best view or spend the best evening. It is one thing to come to a city with your travel guide, it's a completely different thing to know a local right away. Additionally, I love the idea of opening your home to a stranger and to be completely unbiased on who will come over.