The objective is simply to get a proper spread of densities across the print. A problem is that different papers give different visual impressions for similar densities. A familiar example is that the DMax black on a matte paper may actually look blacker than the black on luster paper, but it measures much less dense. Similar things happen in the highlight areas. Then there is the problem with developers. The curve produced by one developer may not be the same as another; so matching a particular zone to a density may have varying affects for the rest of the scale. The subject of tonal variation with different developers for modern papers has not been addresses much, and maybe some work is in order. If you use just one kind of paper and developer, you can probably experiment to find out what looks right for you.