A neck dissection with certain types of oral cancer (usually non HPV) is often done as this cancer can be aggressive and seed to the nodes - in the early stages it does not show up on a scan leading people to mistakenly believe they are clean when in fact they are not - and usually end up with a second surgery once a node pops up. Nodes are sometimes precautionary - I've seen it happen numerous times (most recently to someone I know's husband) where they operated didn't remove any nodes or too few only to have the cancer come back with a vengeance. I scanned clean twice before my surgery (one was an MRI) my dr was insistent I was having a neck dissection, node popped up before my surgery. Scans aren't 100 %. Even if it's an early stage cancer how aggressive it is dictates the spread a lot of the time.