There are two reasons that a 'spit' system may be your choice.
First, you may have a relatively new furnace that you don't want to replace so using it in combination with a smaller geothermal unit might suit you best.
Second, you may find the cost of a geothermal system sized to provide total heating and cooling in your house prohibitive in which case a smaller system sized to provide your total cooling needs but not all your heating may be an optimum choice.

A split system includes a normal gas furnace with its air handler for the forced air distribution. The geothermal heat pump provides heating and cooling via a coil in the air stream in exactly the same way as a conventional air conditioner does. Usually we will size the geothermal unit to handle all the cooling demand and all the heating demand except for those infrequent very cold spells which can occur in this area. In the very cold spells the system reverts to using the gas furnace. This means that the geothermal unit can be significantly smaller and therefore cheaper but it still provides all the cooling and perhaps 80% of the heating. The payback period for this type of system is usually shorter than if the geothermal is sized to provide all the heating.