Phil has pretty much covered it, but the "reason" to do this is that naptha is generally quite aggressive to the styrene plastics used in scale models, essentially melting the surface so that the pigment is well bonded to it. This may also produce a thinner coating, since as Phil pointed out, you are removing a fair amount of binder.
As a practice, why bother? There are better ways to accomplish the same thing (Alclad, for one).
If you use naptha, use it as a thinner, keeping in mind that it has a lower flash point than most of the other common solvents used for thinning paint.