Priming sprays a thin grip-and-fill undercoat — sold in this hobby as “surfacer” — over bare plastic before color. It does three jobs: bonds paint to slick styrene so it does not chip off, unifies part colors under one neutral base, and reveals surface flaws while they are still fixable.
If you are painting, prime. The one exception is an unpainted snap-fit build with panel lining only — bare plastic needs no primer for that.
Go deeper in our paints & primers guides.