What is actually holding the building up
A pier is a small, isolated footing set below grade under one point, usually a post. It commonly gets formed with a round tube, a sonotube, or poured into a hand-dug or augured hole down into firm subgrade rather than resting on topsoil. In post-frame construction the posts run from the roof trusses down to the ground, and each one lands on its own pier instead of a continuous footer running around the whole perimeter. Load travels down the posts and lands on the piers, so a handful of small footings under the corners and bearing points is doing the work that a continuous footing would do under a wall.
That is different from a slab foundation or a stem wall, where the floor and the walls sit on one continuous footing carrying the whole structure at once. Because piers only carry what lands directly on top of them, the floor between them is genuinely a separate question. A lot of pole barns go up, get roofed and sided, and sit on packed gravel or dirt for a season or two before a floor gets poured, and nothing about the piers changes when that floor finally goes in.
The pier count and the spacing between them follow from where the posts land and how much weight and span the structure is asking each one to take. Those numbers come out of the framing plan, so have that plan with you when we walk the site, and none of it gets guessed at with an auger already running.
Where piers alone stop being enough
Piers say nothing about what the floor itself has to carry once it goes in. If the building is going to store hay, park a truck or a tractor, or take a loaded trailer tongue set down hard, that floor needs its own thickness and its own base under it, the same as any garage or shop slab. A pier holding up a post does not spread out a wheel load or a stack of round bales sitting on a few square feet of concrete. Those are two different jobs, even though they end up in the same building.
Ground movement matters too. Frost heave, the ground swelling and settling as it freezes and thaws each winter, can work a shallow pier loose or tip a corner of the building out of square over a few years. How deep a pier needs to sit for the ground it is going into is exactly the kind of thing worth walking through before the holes get dug, not after the posts are already standing on it.
Pouring the floor once the posts are already standing
A good share of the shop and storage floors we pour go in after the building is already up, posts, roof and siding all in place. The floor gets formed and poured around the posts, thickened and jointed the way any shop floor is: a compacted stone base under it, a mix suited to what the floor will carry, and control joints cut on a grid so the concrete shrinks where we put the cut instead of wherever it wants to.
That is usually a job for our line pump rather than a chute. A ready-mix truck can only reach so far in through an open end or a door, and past that somebody is pushing wheelbarrows around posts that are already in the way. We run 2.5 and 3 inch line off our trailer-mounted pump instead, so the truck stays outside on solid ground and the concrete goes exactly where the floor needs it, at one steady pace instead of in fits and starts.
Whether the piers are still ahead of you or the posts are already standing, tell us where things are in that sequence when you call. A building already up changes how we get the concrete in, not whether we can pour the floor.