I would find it much easier to get flight/fare info from Chicago to New York just by inputing a request for CHI-NYC just once and getting all flights/fares. Otherwise, I would have to input seperately ORD-JFK, ORD-LGA, ORD-EWR, MDW-JFK, MDW-LGA and MDW-EWR. (I'm not sure if they're included in the NYC designation, but also ORD-HPN, MDW-HPN, ORD-ISP and MDW-ISP.)
It just makes it easier.