Well, for example, suppose you only had the Delta-9 and nothing else, that would raise severe questions about the sample. You want to see the parent drug, the 11-Hydroxy right off, you know it is not going to be there, and you want to see the Carboxy-THe. That tells you the drug was consumed by the individual and it was being metabolized.
So you have a good handle on its use and one verifies the other. Much the same as we do in a tube biologicals like blood and urine. The reason for that is it avoids any question was it contaminated or mixed up.
If we find it in the blood, we want to find it in the urine because one validates the other.