What it does in a cold pour
Cold weather slows concrete down. The chemical reaction that turns wet paste into hardened concrete runs on heat, and when the air and the ground are cold, that reaction crawls. Calcium chloride speeds it back up, which shortens the time before a slab can be finished and shortens the risk window before an early freeze can damage a fresh surface.
It has been used this way for a long time because it is cheap and it works fast. That is exactly why it gets reached for on a cold morning without a second thought, which is where the trouble starts.
Why it is the wrong accelerator near steel
Chloride ions promote corrosion in embedded steel. Rebar, wire mesh, anchor bolts, anything metal cast into the concrete sits inside that mix for the life of the slab, and chloride worked in around it is chloride that stays. Corrosion is slow, but it is not reversible, and it shows up years later as rust staining, cracking along the bar, or worse.
That is why chloride based accelerators get kept away from reinforced work. Since most flatwork carries wire mesh or a tied rebar grid, and foundation work always carries steel, calcium chloride is a narrower tool than its price suggests.
What takes its place
Ready mix plants also offer non-chloride accelerators, which speed the set the same way without carrying chloride into the mix. They cost more than calcium chloride, and on reinforced work that is the trade worth making.
Which accelerator, if any, belongs in a given mix is a plant and mix design question, decided against the reinforcement and the exposure of that specific pour, not a default answer for every cold morning.
Timing matters here too. An accelerator shortens the window before a slab can be finished, which can be the difference between a crew working a surface on schedule and a surface starting to set before the finish is on it. That is a good problem to plan for on the phone rather than discover with a crew standing over a slab that is setting faster than expected.