When should one use a concrete plasticizer? Should it be used for internal floor slab?
You use plasticiser when you need to improve the workability of concrete.
Concrete's workability improves as you add increasing amounts of water. Increasing the amount of water in the mix beyond the optimum leads to weaker concrete. Insufficient water also makes concrete weaker, as it is harder to compact it to remove air voids.
For a normal floor slab, I wouldn't expect to need plasticiser to get the concrete sufficiently workable.