Air handlers and furnaces aren't often used side by side in Denver. If you use a furnace, you probably don't need to think about an air handler.
Air handlers are typically linked with heat pumps and manage airflow around your house. Some systems also provide secondary heating and cooling parts to support the heat pump.
A furnace works differently by sending warm air into your vents, which circulate it across your home. Since furnaces have combustion chambers to make hot air, they don't need some of the pieces you'll find in a standard air handler.