Dedicated circuits are required by the National Electrical Code, but what are they? The code requires that every fixed appliance must be served by a seperate, dedicated circuit, not shared by any other appliance.
A dedicated circuit is defined as - a circuit that serves only that appliance. These appliances include refrigerators, freezers, electric range, electric water heaters, furnaces, washers, dryers, microwaves, dishwashers, and garbage disposals. And don't foget about heat pumps, sump pumps, and air conditioners.
A good rule of thumb is that if it has a motor, it needs its own circuit. I'd rather have too many circuits, rather than not enough!
If you've ever had a major appliance connected to the same circuit as another and a circuit breaker has tripped or a fuse has blown, then you understand the need for these specific circuits.