Additional facts:
A test cross between an organism whose genotype for a certain trait is unknown and an organism that is homozygous recessive for that trait so the unknown genotype can be determined from that of the offspring.
Answer:
You'd use it when you wish to see if the one that you think is a homozygous dominant is a purebred, because if you did get a recessive offspring then it proves that the animal is a heterozygous and not a purebred.