Breathing is improved, compression loss from relieving is reduced. The port transitions into the relief and deck more smoothly.
The Type 4 early H-D "W" engines (separate transmission) have both angles machined into the tappet blocks (the crankcase is "normal"), the later "K" engines (unit transmission) have concentric tappet blocks with the angles in the right crankcase locations.
All have horizontal cam journals and beveled lobes complementing the cross-plane angles. The "W" engines used solid flat tappets, while the "K" engines used roller tappets, so the lobe shapes are very different even with similar durations.
Many later automotive flathead tuners identify the "K" especially as an influence.
