A couple of additional thoughts:
Primer provides a surface for the finish paint to adhere to, or "bite". It also gives you a base color. You can get slight differences in the finish color with different colors of primer. This is truer for colors that are more translucent, such as pearls and light colors. Primer will also help to cover some minor imperfections, such as light scratches.
Orange peal is caused by a combination of things. As mentioned above the paint should "flow" out on the surface, or "lay down". The wetter the paint is the better it "lays down". However, if it is too wet it will run and that is much more difficult to fix. The other cause of the orange peel is the air. When you spray paint it is atomized into thousands of tiny droplets that should join together, "flow", or "lay down". The same air that atomized the paint into little droplets also prevents them from flowing together.
Take a little paint and apply it to a surface. Then blow into it with a straw. You will see the paint form a circular depression due to the air forcing the paint off to the sides. That depression, multiplied by thousands, is "orange peel".
Sometime you can have perfect atmospheric conditions, i.e. temperature, humidity, and get the amount of thinner or reducer exactly right, and the paint will flow together before it gets too dry. With enamel you can let your last coat get tacky and then spray on a very light "mist" coat of thinner to keep the top coat wet enough to lay down without orange peel. This is tricky, so if you try it be forwarned that the results can be fantastic, or a disaster!