The discussion may involve-oh no!-chemistry!
People who prime a model, whatever material it's made from, to help improve the adhesion of their color coats. It also can help if you like to inspect your model as you proceed through assembly, by highlighting scratches, sink marks, and similar flaws that aren't as easy to see in the unpainted material.
Some people don't. There is no right or wrong way, there is what works best for each of us.
Now, I mentioned chemistry, because some paints work better for primers than others, and depending on the material the kit is made of. Lacquers make good primers for styrene, for example, because they're "hot" and can adhere well to the surface. Water-based acrylics, on the other hand, might not provide as durable a primer coat, because they probably won't "bite" in the surface as well. Some figure companies sold flat enamels, back in the day, for their white metal figures. Again, everyone will have his own experience.
There's also the question of whether to clean the parts. Again, some do, others don't. Years ago, it was generally so that companies used mold release agents, to allow their sprues to pop out of the dies more easily. So the kit came to you with some residue of those mold release agents. And many modelers washed the kit sprues with warm water and a degreasing agent, to ensure as clean a surface as possible. It's not really necessary with styrene kits today, but it doesn't hurt, either, if you choose to do so. Resin kits might require it, and this can vary from one manufacturer to another, depending on their process. Even white metal kits could stand a cleaning. Again, it might not be necessary, but it can't hurt if you do it. But how clean the surface is can affect how well paint adheres, whether it's a primer coat or finish colors laid down without primer.
I'll add that I generally clean the sprues when I start, but even with a clean surface and a primer coat, I've had Tamiya tape take off the finish color layers and the primer underneath. You just fix it and move on.
But there's no rule that says "You must prime everything you build" or "You must wash every kit you build!" There is everyone's experience, much of it shared, but a lot of it unique to to the modeler.
I don't worry about what others do. I'll observe it, note it, give them a thumbs-up of encouragement, and proceed as I have learned to do since I built my first model at 5 (a Model T, assembled with Duco Household Cement).