Why a crew would want more time, not less
Concrete starts its set the moment cement and water meet, and heat speeds that reaction up. A hot day can turn a load stiff and hard to finish well before the crew is done working it, especially on anything with a decorative or exposed finish that takes longer to get right. A retarder is ordered ahead of time to hold that set back so the finishing window stays open long enough to do the work properly.
It also matters on a pour big enough that the first concrete placed and the last concrete placed are separated by real time. Without something managing that clock, the first section can start setting before the crew has even reached the far end of the job.
What it changes on the order, and what it does not
Set-time admixtures belong on the order with everything else, the strength spec, whether the load is air-entrained, whether it is going through a line pump. Anything that slows the set changes when finishing can start, which matters for scheduling the crew and, if the load is being pumped, for how long that concrete can sit in the hose before it becomes a problem.
What a retarder does not change is the final strength the concrete reaches once it has cured. It moves the timeline, not the number on the strength test.
The one thing it is not a substitute for
A retarder is the correct way to buy time on a load that is setting too fast. Water added at the site is not. Adding water to loosen a load that has started to stiffen trades workability for strength permanently, and that trade never shows up until the slab has already cured and it is too late to fix. If a load feels like it is setting too quickly, that is a conversation with the plant, not a hose.
Where this comes up on our own work
We spec the mix before a job is ordered, and set time is part of that conversation whenever the weather or the size of the pour calls for it. A hot afternoon driveway or a foundation wall poured in lifts are the kinds of jobs where a retarder is likely to be on the ticket. We would rather build that into the order than have a crew racing a load that is setting faster than the work in front of it.