It must evoke emotion. A good cartoon should make people laugh or make them mad. Always, the cartoon must give a point of view. The cartoon may be looking at the truth, but it usually is coming from a specific viewpoint. When we look down at an object, the viewpoint is very different when we look up at the object.
Editorial cartoons are the same way.
The Independent doesn't always agree with the viewpoint of each cartoon in the paper. Most certainly the readers don't always agree with them. But we all should agree that political cartoons are thought provoking. Just like a photograph, a well-illustrated editorial cartoon can be worth a thousand words. There probably are about 100 newspapers, give or take a few, that employ full-time cartoonists.