Some colours don't cover well and you will get a better finish laying down several thin coats vs 1 thick coat of paint.
This is especially true for brush painting, where you also vary the direction of the brush strokes.
Most colours in real life are also applied in several layers cars, metallics can have upto double the layer of non-metallic colours.
Another reason is that if the paint is too thick it can take a long time to dry and can develop runs and tears too or flow into small detail areas and obscure them.
I think it will really depend on the type of model you are building and what effect you want to achieve.
A Car finish will be done more careful than lets say a dirty WW II tank.