You may be right but domestic connections are very different from multi-city and typically price differently. It may well be more expensive to add them in after the event (due to change fees or repricing but is worth checking if you have not already done so) but including them within the original itinerary can add less cost than buying them outright.

The obvious advantage here being that you get more protection for late arrivals and departures and consistent baggage throughout your trip with no additional charges to worry about.