I think the easiest way to explain the benefits and logical reasoning for buying a multi-stage furnace is to compare the furnace to a car. A multi-stage furnace gives you many levels of heat (low, medium, high) similar to 20 mph, 40 mph and 60 mph in your car. A single stage furnace only gives you 60 mph all the time, even when the temperature is 50 degrees outside and you only need low heat to reach your thermostat setting. A single stage furnace wastes enormous amounts of gas and leaves your home uncomfortable whenever low and medium amounts of heat are necessary (approx 90 % of the heating season).
A multi-stage furnace uses far less gas when it heats your home in low and medium and with longer run times leads to increased comfort in all areas of your home.
You would never drive your car in City traffic at 60 mph and slam on your brakes and then back to 60 and so on, so don’t settle for single stage heating when it is time to upgrade.