Advantages are not limited to headroom for standing upright since average male is 5’-10” tall and females much less, and the vast majority of population falls under H2 ProMaster height of 6’-4”, even after losing a few inches for insulation.
Headroom for a mid-coach drop-down bed that most men can still walk under when raised is an obvious plus, but there are more design options beyond that if H3 is used fully.
For example, a fixed rear platform bed can now be higher so that garage below can accommodate bikes and other larger gear while bed above can provide room to sit. That’s something few can make work in H2. New H3 height should make it easy.
Extra available ceiling height should also make fixed bed over a dinette possible. Similar to Class C bunk over cab seats, the same can be designed at rear of H3 ProMaster (though longitudinal). Sleeping for family of four should be easier without incurring cost, complexity and compromises of pop-up roof additions, or building makeshift beds below. Larger upper cabinets for added storage will be another plus, etc.
Those who don’t need extra volume can select smaller H2 van, but I predict many buyers will buy the extra room offered by Super High Roof (H3). It may be hard to pass on over 50 cubic feet of extra cargo volume.