If you spray the paint on in a misty fashion, it will not dry smooth, and end up "matte" looking, and require a lot of finish sanding to get it smooth- and then you are not much better off than putting it on with a brush or roller.

NOTE: A slightly less than perfectly smooth paint texture may actually be desirable if your surface has a lot of defects and dents in it that you don't want to take the time to do a lot of body work on. If the paint is perfectly smooth and shiny- you'll in fact see every ding and dent in the surface that you've left. So in the cases of cars with lots of dings that you don't want to take the time to remove each one, mist the paint slightly by putting on lighter coats that don't self-level so much.

In most cases, however, you want the end result to be shiny and smooth, and this is the case if you've done really nice prep work and repair.

If you spray the paint on too heavy, it will be smooth-- AND runny, and you'll have drips on vertical surfaces.

If you do it JUST RIGHT, the paint will look WET and will SELF-LEVEL-- i.e. the surface will "run together" and create a nice smooth surface, that will require no sanding, or little sanding if you are going for a glass smooth surface.