Primer is a "tie coat" designed to really grip a substrate and let top coats, get a grip on it. Top coat paints generally have too much "surface tension" to work well as a tie coat, though some are better than others, none are as good as a real primer. Lastly, primer is easily sanded, so you can fine tune the surface before the shiny stuff goes on. Top coats can also be sanded, but need to be recoated after, just to get the shiny back. I always use a primer, as it's cheap insurance.