Not OneHundred percent Pro.
Done the odd car, car parts and mainly doing helmets and bicycleframes.
Flattened as in sanding down yes... depending on how smooth the primer was applied and if there are any irregularities in it. (cut it down with Grit400 sanding paper or a 3M Scotchpad)
Filler Primer is for a better adhesion of the basecoat, something to grab on.
To smooth things out (quick filling), like when there's no reason for a thick filler putty over a repair area. To seal things off, like putty sucks in more paint then a sanded-over O.E.M. paint area. It evens things out.
On a final note, give it some quality time when prepping!
Proper prepping always shows through in a paintjob. Might be hard graft or it appears like going nowhere but do it!