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Coefficient of drag (Cd) is dimensionless and is a way to access the design of a vehicle, and compare that to the design of any other vehicle.
CdA has a dimension -- units of area, and is a way to know the actual drag of a vehicle, and compare that to the drag of any other vehicle.
Reducing either the Cd or the area helps improve the efficiency of the vehicle. But if the Cd is say about 0.33, you have to reduce 3X as much to equal a reduction in Cd.
For example, if a car has ~25 sq ft of frontal area, and the Cd is 0.20, that is a CdA of 5 sq ft. To reduce the CdA down to 4 sq ft, you can either improve the Cd down to 0.16 or reduce the area down to 20 sq ft. The Cd is much easier to improve -- reducing the frontal area by 5 sq ft would require a huge amount of work.
Knowing both the Cd and the CdA are very useful, in my opinion.
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