If the reactions of every organism were different, biochemistry would be a poor science. Contemporary biochemistry depends on the ability to extrapolate information from one system to another. For example, if humans and animals made cholesterol in fundamentally different ways, scientists would have no way to find a compound to treat high cholesterol and prevent heart attacks. It would be impossible (and unethical) to screen the millions of known organic compounds in humans to find an effective treatment. On the other hand, using enzyme systems, researchers can screen many thousands of compounds for their ability to inhibit an enzyme system in vitro.
They can then screen the small number of active compounds for their effectiveness in laboratory animals, and then in humans.