NPV and specific are two entirely different concepts. You should draw a 2x2 table to see the difference. NPV is useful clinically because in clinical settings, you always start with a pre-test probability (which is informed by prevalence). You do the test and the NPV of that test will govern how much the scales tip in favor of or against your diagnosis. Specificity doesn't have that property. Specificity just measures how many people without the disease don't have a specific marker. So say you have a specificity of 100%, i.e. healthy people don't have elevations in marker X. You think your patient has disease X so you do a test for marker X. It comes back negative. How much does the scale move? You don't know. You don't know because it could be true that even though 100% of healthy people don't have the marker, only 1% of the diseased people have it. So doing the test didn't really tell you much.
That's why a high specificity helps to rule in disease. If your test is positive, that means that you likely have the disease because high specificity means that healthy people are not likely to have elevations in that marker.
