When a group is formed, normally the travel package will include round trip air travel from the airport nearest the group leader's home, as most group leaders will bring along participants from their area. Group members generally want to leave and return together.
It is also true that people living far away from the majority of the group will book into the tour. Occasionally, someone will want to fly Business/First Class, especially on long international flights. Some hope to save on the cost of the tour by applying earned air miles to their airfare. I have also had tour participants embark on back-to-back tours requiring flight arrangements from one tour's final stop to the next tour's starting point.
Offering a "Land Only" rate allows for some flexibility to accommodate the above situations. This rate still includes all of the group's accommodation, meal, activity, services and entrance fees in the tour destination. Air is then handled separately for those individuals requesting special flight arrangements.