Why a foundation needs a step up before it needs a floor
On a crawl space build, the stem wall is what stands between the footing and the framing above it. It holds the sill plate the framer builds on, and it leaves open, unfinished ground underneath the house rather than filling that space with a poured floor. A footer and stem wall foundation lets the shell go up first, on schedule, while a floor inside it can wait.
A stem wall also does the job of getting a building level on ground that is not. A lot that falls corner to corner still needs a flat, level line to frame from, and a wall of varying height around the perimeter, tall where the ground drops away and short where it does not, is what gets a sloped pad to a single level plane without hauling in fill everywhere.
What goes wrong when a stem wall gets skipped on the wrong pad
A pad that drops noticeably from one corner to another and gets a monolithic slab instead of a stem wall forces a bad choice later. Set the floor at the low corner and the building sits at the low point of its own lot, collecting water off the roof and the yard. Set it higher and the approach into the building turns into a ramp steep enough to be a problem every day it is used. A stem wall avoids that trade by letting the wall itself take up the difference in grade while the floor stays level.
Backfill timing is the other place a stem wall gets damaged after it is built right. A wall picks up much of its real strength once the floor system above it and the slab or footing below it are in place, and dirt pushed against a green wall before that, especially on the side with more fill to hold back, can crack or bow it. That damage is avoidable and it happens because somebody wanted the site cleaned up before the framers showed up.
The hardware has to be right before the wall goes green
Anchor bolts and any hold down hardware go into the top of a stem wall while the concrete is still soft, set to the layout the framer's plan calls for. That window closes fast, and once the wall has cured, moving a bolt means drilling into finished concrete instead of setting it while the pour is still workable.
That is the reason the framing plan needs to be settled before the wall gets formed, not the week after. An opening or a hold down location that moves after the pour does not take the hardware with it.