Medan is a huge city (it's the third largest city of Indonesia) and most backpackers use it as a thru-point: it is understandable as there is not much to do /see and most people do not speak English.
It is difficult to skip it if you are coming from Banda Aceh and it is there that you will have to catch a bus to reach some more remote places in the center of Northern Sumatra.
Medan is the capital of North Sumatra and the third-largest city in Indonesia. It's a commercial hub and a typical big Asian city. Pollution, trafic and poverty mixed with buidings, malls and air conditioning. For tourists, it's the perfect gateway to North Sumatra and its jungle.