By comparison, here's part of a test that Fred Newman made for me a few weeks ago, of Ilford HP5+ developed in D-23 1:1 (not divided D-23).
Note that this is a more diluted developer - and D-23 has a reputation as being low-energy - so the contrast range is much lower - but how much lower ? There's no guess work here.
To get a normal Contrast Index of 0.5, we need to develop for 10 minutes. Also note that even after developing for 16 minutes, we still haven't reached N+2 expansion. This film/developer combination is probably great for a sunny day at the beach, but not for bland subject matter.
On the other hand, with this film/developer combination, N-2 development happens at around 5 min 15 seconds.
So not only do we know that this is a "lower contrast film/developer combination", we know exactly how low, and how long to develop it to get the contrast we want.
There's no guesswork involved. Instead of relying on someone's well-meaning but vague advice (Person A: "I've been souping Plus-X in X-Tol 1:8 for the last decade. I really like it." Person B: "You gotta try Tri-X in PMK-XYZ at 3:2:1000. Nothing beats it") we get unambiguous facts we can really use, and a common language with which we can easily compare and categorize film/developer combinations.
This isn't the whole story, though. We know that different film/developer combinations give different effective film speeds. But which one gives which speed ? Shall we rely on friendly advice ? ("Try pushing FP4+ to ISO 8000 and develop it in Nescafe - you gotta see it to believe it")