I’m not sure you miss anything. I used to primer all my models (armor/aircraft/ships/cars) in basic gray primer. This highlights irregularities in the surface. But in reflection, ships, cars and aircraft are supposed to have smooth surfaces, whereas armor not so much. So I don’t bother with standard primers anymore. I chose instead to use either a very dark grey (basically black with a bit of white added) or a Burnt Umber brown color. For German Armor, as you indicate, a very dark red, which is Burnt Umber with a touch of red added for my color choice. I mix these paints in enamel or lacquer, and thin them with lacquer thinner, which gives an added “bite” to the finish, ensuring that whatever colors I chose (acrylic/enamel/lacquer) won’t easily come off. So in answer to you question, I don’t bother with the “primer” coat, I always start with a darker “base coat”.
VR, Russ

I prime my models with grey autobody primer from a rattle can - it snugs down nicely and grips well on plastic and brass PE so my "colour" coats don't peel off. Then I do the black pre-shade with my airbrush and then add colours in thin coats (several shades made by mixing various amounts of buff to lighten the paint) to get my modulation effects. But I wouldn't skip the primer because I've had too many experiences of paint not adhering well and ruining the final result.

Just my 0.02 [insert currency here] worth...