Some NPRs start as pre-TN civilizations and they will not advance further. They will have no TN industry to speak of, but they are easy to conquer, generate wealth, usually have homeworld like mineral deposits (all TN elements in good accessibility but moderate quantity) and, above all else, have different environmental tolerances than your race. This is especially important if they are from low gravity planets, as using them is the easiest way to colonize moons/dwarf planets below your own gravitation tolerance - and those objects are numerous.