You'd have to try to download and read the user's manual, usually available on their website and read the theory of operation section. Often, without the smart control, modulating furnaces start at a low-fire level, and after time increase the level (often to max) until the house reaches the setpoint (often just a two-step, low/max). A smart control often 'learns' the house's responses to a call for heat and can set the specific heat outlet level to make the house the most comfortable with the best economy. The extra cost may be in the need for running a new cable from the thermostat to the furnace. It might also take some time to program the thermostat. This is all generic info - you need to get the specifics. My guess is that the installer is not well trained on the furnace and just doesn't understand. Read the installation manual and ensure it is done per that manual if you want the best performance. There are lots of little things that they can omit to save time/money that really do affect your overall satisfaction. One thing is the flexible break between the furnace and the main duct - this is usually specified and decouples noise from the furnace into the ductwork. There are other, similar things.