Air handlers and furnaces aren't typically installed together in Cleveland. If you have a furnace, you likely don't need to think about an air handler.
Air handlers are frequently used with heat pumps and deliver airflow across your residence. Some systems also offer secondary heating and cooling components to help the heat pump.
A furnace works in a different way by sending hot air into your vents, which send it around your residence. As furnaces have combustion chambers to produce hot air, they don't need some of the pieces you'll see in a regular air handler.