Why concrete shrinks as it cures
Fresh concrete gains its strength through a chemical reaction inside the mix, not by drying out the way a coat of paint does, and that reaction draws the paste in a little as it hardens. Pour that mix as one unbroken sheet and it has nowhere planned for that small amount of shrinking to release, so it finds its own place to crack instead.
That is the entire reason control joints exist. A joint cut early, within the first day, gives the shrinkage a straight, planned line to follow instead of a random crack across the middle of a driveway or a patio. A shrinkage crack that lands right on a joint is the joint doing exactly what it was cut to do.
Reading it as a defect when it isn't
The most common mistake with a shrinkage crack runs one direction: a homeowner spots a thin line along a joint, decides it means the pour went wrong, and starts asking about replacement for a slab that is doing exactly what concrete does. A shrinkage crack that follows a joint, sits flat, and does not catch a fingernail is not a defect. It is normal curing, and it is one of the more common calls a concrete crew gets asked about for no reason at all.
The check is simple. Run a hand across it. If both sides sit level and the crack tracks a joint line rather than wandering across open concrete, that is shrinkage doing its job.
Calling everything shrinkage, including the crack that isn't
The opposite mistake costs more. Because true shrinkage cracks are so common and so harmless, it is easy to wave off every crack the same way, including the one that is actually a sign the ground underneath has moved. The line to watch for goes the other direction from a joint: it wanders off across open concrete instead of tracking a cut line, it is wide enough to notice with your finger, or one side of it does not sit level with the other. Any of those, and ordinary shrinkage stops being the likely answer.
Joint spacing plays into this too. A joint cut too shallow, spaced too far apart, or cut a day late does not give shrinkage a strong enough line to follow, and the crack wanders instead, which can look worse than it is even when the cause is still just curing. Either way, the fix is the same: look at the crack itself rather than guess from habit.