I do not always, mechanically, prime a model. There are good reasons to do so, but also many times you do not have to.
For one, a GOOD QUALITY paint will usually adhere well to bare plastic. I use a lot of lacquers, acrylic-lacquers and enamels and all of those have a very hard, durable finish and bond well to bare styrene plastic. I find paint not intended directly for models (like the $.99 water based craft stuff) has poor adhesion (and coverage) so a primer is needed there.
If I have a kit with a mix of styrene, resin, photo-etch metal, etc. then I may prime that to give a smooth, neutral, base coat for the paint to adhere too. Plus paints often don't adhere as well to resin or metal as they do plastic. I also prime a kit if I have to do a lot of sanding, putty work, etc. Just to make sure the seams are nice and smooth, etc.
You should prime a kit if its molded in some icky color like black or yellow, and you want to paint it, say, white. The white paint by itself will not cover a dark plastic very well without many, many, coats. Likewise, bright plastics will bleed through light colored paints.
Some people prime models and wind up with thick gloppy layers of primer and paint, making the kit look bad. You don't want to obscure details on the model with too much or too thick of a primer.
You can also use a base coat of paint as a primer. I do that a lot... On an armor kit, for example, I often spray a good bit of the kit a dark grey-brown base coat of paint. Then I apply the camoufalge paints on top.
Everyone has their own favorite primer. I like Tamiya fine white and Tamiya grey surface primers. They come in spray cans and go on super smooth and thin. The grey stuff has a bit more tooth and the fine white is super smooth. You could use it as a flat white paint finish.