Many flights do this; they are routed through two or three cities via a hub. In many cases with international flights the flight changes equipment. CO is not the only airline who does this, they all do. By putting the two flights under a single flight number, it shows in reservation systems as a direct flight with a stop, which puts it up at the top of the list right under any nonstops but before connecting flights. It's a marketing ploy to improve the positioning of the flight in the reservation systems listings. Many frequent fliers dislike these flights because the airline awards mileage and elite points as if the flight was one flight, not a connection. So a passenger flying IAH-EWR-FRA gets one elite point in each direction, not two, and miles as if flying IAH-FRA nonstop (which does not exist on CO) as opposed to miles calculated as IAH-EWR and EWR-FRA.