What the tube is actually doing
A sonotube is a form, nothing more, standing in for the wood boards used to form a footing trench or a wall. It gets set into a dug hole, braced plumb, held at the depth the job calls for, and backfilled around the outside to hold it in place before anything gets poured into it.
Concrete goes in and gets worked, rodded or vibrated enough to close air pockets against the cardboard so the finished surface comes out solid rather than honeycombed. Once the concrete has taken its own weight, the part of the tube above grade gets trimmed or peeled away. The part below grade has already done its job by then, holding the shape while the concrete set, and it is left to sit against the finished concrete underground.
What goes wrong when the tube is not set right
The bracing is where most sonotube trouble starts. A tube that leans even a little while it is backfilled and braced pours a footing or a post base that is off center at the top, and that throws off every measurement built on it afterward, a deck ledger, a post cap, a beam line. Getting it plumb before the truck shows up matters more than almost anything else about the pour.
Depth is the other one. A sonotube footing carries the same rule as any other footing, it has to bear below the frost line at that address, which is a code question tied to elevation and set on the estimate before anybody digs, not a guess made on site. A tube seated too shallow, or seated in loose spoil rather than firm undisturbed ground at the bottom of the hole, moves with the frost the same way a shallow footing anywhere else does.
A tube left standing above grade after the pour, never trimmed back, is the other common miss. Cardboard exposed to weather stays wet longer than the concrete around it, and it looks unfinished for as long as it sits there. Trimming it flush is a small step and it is easy to skip on a job that has moved on to the next thing.
One tube, several different jobs
The same tube forms a deck post footing, a porch column base, a mailbox post, or a fence post, since all of those are an isolated point of support rather than a continuous wall. What changes from job to job is the size of the tube and how much load lands on top of it, and that comes off the plan for structural work rather than off a guess.
A sonotube is the form. What it leaves behind, once it is stripped, is a pier, and how a pier compares to a continuous footing is its own question. Our pier glossary entry covers that comparison.