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And not just put a coat of the foundation colour directly on?

I've read quite a few threads where people say that you still need to prime the models first, even if you're airbrushing a layer on afterwards, but I've never read anyone say why.

I've just had a session with an airbrush, mostly models which were primed black, but I had one model that I recently stripped and not primed yet, so I sprayed it, to see the results and difference between the two.

It's a nice solid coat, it's not flaking, and it's visually identical to the primed ones. So, why prime?

Priming provides a layer that sticks to the model and provides a surface that paint WANTS to stick to (you can't rub a primer off the model with your finger when it's dry).

It'll make the paintjob stick better.

I don't prime when airbrushing.

I do my base coat, then a few layers of zenithal shading, then hit the whole thing with a layer of Satin Varnish. The point of priming a model is to make the basecoat layers stick, and to protect them. I find varnishing after doing the basecoating does the same thing, and I've had good results with it so far.

I find primer is a must if you want paint to stick. Otherwise it rubs off on my fingers at the lightest handling.

It also provides an even base in case of hybrid models (metal and plastic, for example,) otherwise the difference in the base material can show through.

Of course, there's no need to use spray primer now that Vallejo makes those nice airbrush primers in several colours.

I only use primer on metal and primer for the above reason having the color coats stay on the kit. When building plastic I shoot enamels thinned out with lacquer thinner. This is very tricky to use because you can melt your model but I like it because it "burns" the paint into the plastic. Doing my airbrushing this way makes sure the paint stays on the build.