The contractors usually stick to the method they knew. Put primer to ensure color paint stick and cover those unevenness and stain on wall. The reason is simple; they do not want to risk doing the same job twice if the owner is dissatisfied.

Nowadays (starting like 10 years ago), the paint has been improved and many claimed no primer required. I did not see Jotun saying no primer needed (maybe I missed) but stated that the paint (usually the acrylic paint) has good hiding property and cover unevenness very well.

I'd just go straight with color, 2 layer, and done.