Which part does the "wearing"?

The parts need to be the same hardness. The break in establishes a mutual surface roughness that lets the oil protect the individual faces. If at any point down the road the oil isn't between the parts, the surfaces get damaged, which makes it harder for the oil to continue to protect the surfaces. And then they both scrub off. You want "just enough" spring pressure at the break in rpm to allow for the surfaces to wear together, and once that's set, you want enough to control the valve motion through the entire rpm range. That's why the instructions say to remove the inner springs.