like lewbud said, price is a factor....but more importantly is it simply works better. I can take most acrylic based paints off in less than five hours with it. That means after a good rinse in the sink I can be applying a new coat of paint on the same day I messed up the first coat.
As far as primer goes, I have experiemented with quite a few. I found the automotive primer too inconsistent. Sometimes you get a perfect coat smooth as glass, other times you get a runny mess. Lewbud mentioned Tamiya Surface Primer.....that stuff is like the holy gospel of primers. Goes on perfect everytime, dries quickly, self levels, lays down a perfect flat finish. Awesome stuff. When doing aircraft, I will use Tamiya White Primer for my white undersides and not worry about applying another coat of white. Also, the Tamiya Grey Primer is a near spot on match for Light Ghost Grey. The stuff is easily worth it's 9 dollar price tag