NAPS is typically offered to candidates who show great potential to be successful midshipmen and officers, but are considered not quite ready for the academic rigors of USNA. They are otherwise stellar candidates.
Often — but not always — these candidates are prior enlisted who are a few years removed from high school; recruited athletes; candidates from disadvantaged backgrounds; and so forth. A nomination is not necessary. USNA does not publish their academic stats (GPA, SAT, ACT).
Once they complete NAPS, they are considered fully ready to tackle USNA. That is, on equal footing with direct admits. If anything, they are looked up to by the direct admits, having already taken on a bit of military life.