....1) It provided a well adhered surface for paint to bond to. Styrene and resin are notoriously bad surfaces for paints to bond to, primer is thought to provide (and good primers do) a better paint surface as it is designed to bond to the substrate and then the colour will bond better to the primer. They also even out the model colour prior to paint resulting in a more even finish.

2) Those who chose a bold coloured primer, be it black, white or the final vehicle colour, also use it as part of the final colour process. In the case of black primers, you get an automatic darkening effect near the edges as you do not completely cover the primer with the top coat. This is frequently part of a nuanced and multi-shade painting process that aims to artistically reproduce not the actual painting of a vehicle, but the look of a painted vehicle in certain light conditions.

Pre-shading is part of the nuanced multi-shade process I spoke of above. Again, the aim is, through applications of various shades of the base colour, to achieve the look of a vehicle in natural light. To capture both the physical fading and colour variation in the real paint as well as the variations seen in perceived colour with the naked eye in real life. Again, the idea is that one is trying to capture, through the use of various paints, the variations in colour and wear that the human eye sees when looking at an object that may actually be, provably, only one colour.

It is an artistic attempt to capture reality. Similar in intent to impressionists trying to capture the a subject using quite garish colours sometimes, when the actual object actually has none of those colours in it.

Personally, it's not for me. I will paint a base colour then add weathering to achieve the look I like. I see the pre-shaded/modulated paint schemes as too cartoonish and, to my eye, only looks realistic from one angle of viewing. Something that looks great in photos but less so, to me, in real life.

But there are a great many who subscribe to this more artistic representation of models.
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I pre/post shade single color i.e. olive drab, sand, schemes as a means to bring tonal variety and therefore greater visual interest to what is otherwise a monotone color scheme. Granted it's not "realistic" but more artistic. Used in combination with other weathering techniques it produces different tones and shades that to me make the subject look more interesting. The key is to apply the color coat in very thin layers to allow you to better control the degree of coverage leaving more/less of the preshade to show through.