One pour instead of two
Most foundation work happens in two separate placements. A footing goes in first, with steel and a keyway or dowels left in the top, it cures, and then a wall or a thickened edge gets formed and poured on top of it, often days later. A monolithic pour skips that gap. The perimeter is formed with a thickened edge built right into the same forms as the slab, steel goes into that thickened section, and the whole thing, edge and floor together, goes down in one placement on one day.
That is why a monolithic pour shows up most on garage floors, shop slabs and similar flatwork, where a thickened perimeter is enough to carry the load down to firm ground and a full stand-alone foundation wall is not part of the plan.
Anchor bolts and steel get one chance to land right
A two-pour foundation gives you a natural checkpoint. The footing cures first, and if a dowel or a bolt is off the mark, there is still a wall pour ahead of it to fix the layout before anything is permanent. A monolithic pour does not give you that second stop. Steel and anchor bolts go into the thickened edge while the concrete is still plastic, set on the layout, before the surface sets up.
Miss that step, or set a bolt off the mark, and the fix is not a second pour. It is drilling a wedge anchor through concrete that has already cured. That works, but it is slower, it leaves a visible patch, and it does not hold the same way steel does when it goes in wet on layout. Getting the layout right before the concrete lands matters more on a monolithic pour than on almost any other part of a foundation job, because there is no second placement behind it to correct a miss.
It does not replace a full foundation wall
A monolithic pour suits a shallow foundation, the kind that carries a garage, a shop or comparable flatwork, where a thickened edge under the slab is enough to get the load down to firm ground. A stick-built structure sitting on a taller block or poured foundation wall is a different job. That wall has to reach up from a footing to grade, sometimes several feet, and it is its own placement, formed and poured on top of a footing that already cured.
Folding that much wall into one pour with the slab is not what a monolithic pour is built to do, and treating the two as interchangeable is where a foundation plan goes wrong. Which approach a given building needs comes down to the load, the wall height and what the ground under it is doing, and that is a site-specific call rather than a rule of thumb.
It does not skip the joints either
One continuous pour reads as one solid piece, so it is easy to assume it does not need control joints the way a slab poured on its own does. It still does. Concrete shrinks a little as it cures whether it went in as one pour or two, and if nothing tells that shrinkage where to go, it picks its own line. Joints still get cut on a grid across a monolithic slab for the same reason they get cut anywhere else, to aim the crack instead of leaving it to chance.