Can't we just paint over what is there already? NO!
Do we have to use primer? YES!
But why? It is an extra cost and more labor...
You might have experienced this at some point: You purchase your dream house and walk into the kitchen to start painting the interior that beautiful shade of blue that makes your eyes sparkle. But you begin to notice when you are patching up those annoying nail holes, the paint is peeling. And as it peels, you see the top coat (grey), then some horrible orange color that should never be on anyone's wall and then under that is that rusty beige that matches the "Almond" color for refrigerators from the '70s. Someone didn't use Primer so now the paint is peeling off .
The best option here is to use a primer first so your beautiful blue paint will not be tarnished by what is under neath and so it will stick properly and last longer.
Exterior painting is even more particular. Because materials on the outside of buildings varies so much and we have to consider weather and moisture issues, any surface to be painted MUST be primed first. Primer will protect the material and allow for better adhesion for the paint and increase durability.
Let's look at wood for example. Wood is porous, meaning it will suck up alot of paint in a small area. Primer will seal it from allowing too much moisture into the wood and will allow for a more even coat of top coat across the surface.
To properly paint exterior surfaces, it is always highly recommended to use Primer before painting the top coat. There are additives that can also be added to Primer to help increase moisture barrier. Adding additives in the Primer might change the tint of the paint which is better than changing the tint of your top coat.