The thing about primers is they are made to help protect everything underneath them from rusting or corroding when exposed to other elements such as water, acid rain, and saltwater, etc.
The reason why you need to use a primer is that it creates a barrier between your car and the paint, so any damage or rusting from underneath the body of the car, no matter how small or big, can't be seen from above after you've sprayed on a coat of paint.
That's why we recommend all beginner painters use primers when they're starting out in the automotive painting business. Although it's not cheap, using a primer is much cheaper than having to repair rust damage later on.
Cracks in Body Work
Painting without primers can cause the paintwork to crack within a matter of months, sometimes weeks.
It protects your investment and stops rust damage from accumulating on your motor vehicle; which can cause unsightly corrosion spots that will never be removed unless you sand down the entire body of your car.
The bottom line is, if you want to keep your cars paintwork in pristine condition, the use of a primer is highly recommended before painting.
If you paint a car without primer, the paint will not last. It's as simple as that...