Roasted coffee exhibits different flavors based on roasting methods and duration. Single origin (coffee from one country/region) coffee is generally roasted at light to medium levels to show the inherent flavor of these coffees. You will experience the most flavor potential at light to medium roast levels.  The longer coffee is roasted, more roast/smokey/charred flavor comes out.  Think of it like a steak cooked medium rare vs well done.

Lighter roasts accent the higher acidity levels (think dryness as in white wine) and lighter body/mouthfeel. Medium roasts represent a balance of acidity and body while still exposing the single origin flavors. Dark roasts accent body, heavy to even syrupy mouthfeel, little to no acidity, and often have roast/smokey flavors. Most all of the single origin flavors have been roasted out of the coffee.

Coffee beans roasted to Dark and beyond can literally come from any region and will only provide a heavy, smokey, roasted flavor. Hey, it is fine if that is what you desire. Roasting coffees to this level may defeat the purpose of offering specialty coffee though. It is the flavor profiles available in single origin or blended single origin coffees roasted at light to medium levels that makes them special!