Why Primer? That is a good question, let me see if I can't help out some...
Primer will not only help find un-even sections on a body, it also gives the paint something to adhere to. While you could lightly sand the body and give the plastic some tooth, scratches might show even after a couple of coats. You could have a perfectly sanded body and one area has a scratch deeper than the rest....it will show especially on high gloss auto finishes. So you would then have to go back and prime it anyways...
Primer is a must with some paints....like certain auto laquer colors. It helps protect the frail plastic from withering away or turning into a pile of un-recognizable plastic.
Also priming the model with a color of your choice could affect the overall outcome...for example pearl paints or candy paints with a silver base coat. Another good reason for priming is the model companies don't always mold the plastic in the most ideal colors! Though they have gotten better about it over the years...Do you know how hard it is to paint something white, when the plastic was molded in RED? Virtually impossible without erasing all the detail from paint coat buildup...So shooting a primer coat of another color (I often use flat white/grey) makes the task a lot easier...
I'm sure there are other reasons as well, but these are some of the reasons I prime parts. Don't get me wrong I don't prime every single part...but a vast majority get hit with a rattle can of primer.
I hope this is of some help.