The sacrificial leader up top grants me the license to wire, prune, pinch, etc, the parts of the tree I'll be "keeping" (in the parlance of bnut member BVF, the "keep" regions) for the future bonsai design, while also maintaining full vigor in the tree globally.
It's both a source of vigor (after needles have hardened and it's paying back to the tree) and a sink of vigor (gobbles up stored starch faster than anywhere else in the tree) depending on which phase of growth the tree is in. That's useful because sometimes I want to limit vigor of the "keep" regions of the tree while still having a tree that is so globally vigorous, which lets me quickly close wounds, regrow roots, generate lots of new buds all over the tree, and thicken the trunk or some segment of the trunk line (thickening is often goal #1 of sacrificial growth but it's useful for all "expensive" things that trees have to do when grown as bonsai).
The "keep" regions of the tree are more heavily wired, pruned, pinched, and generally manipulated for bonsai purposes. In that case, the sacrificial leader also serves as a very effective "sink" for stored energy to limit the vigor of the "keep" regions, to make sure that response growth isn't crazy (consider that we're often growing in a larger pot during this phase too -- we need that vigor to go somewhere and we dont want it to go to long internodes/big leaves in our "keep" areas). But the sacrificial leader also makes sure that when we work these keep regions more mercilessly, the tree doesn't get the wind knocked out of it vigor-wise. So it's like an enormous vigor dampener, in to put it in engineering terms.
A typical successful result when using sacrificial leaders is that the leader, especially if it is solo'd out to just 1 to 3 shoots, will grow very large needles, huge terminal buds and very long internodes (just what we hoped for). Meanwhile, the "keep" regions will begin to diverge into the direction of bonsai scale features. For example, it's very typical to have a black pine where a singular ("poodle" style) leader shoot has 5 inch needles and gigantic candles, but the future "keep" pads below have already reduced down to sub-inch needles through successive bifurcations (through decandling and such). Recall that the leader has only 1 or 3 shoots at the top, and perhaps nothing between that and the keep region. Meanwhile, the kept pads might have many many small shoots by now, if this is being done right.
In my personal mental model of conifer bonsai, it's sacrificial all the way down, forever. Every tip is "eventually sacrificial" and the above model works for the entire canopy, to a degree, not just the trunk line. I will grow and style extraneous "sacrificial pads" that extend past the silhouette of my pines / spruces / hemlocks (as long as they don't excessively shade anything below them that is), patiently waiting for interior pads to strengthen to a certain threshold, and then I'll cut them back.