Pharmacologists determine a drug's pharmacokinetic characteristics empirically during clinical drug trials. From these studies, they are able to determine the solubility and distribution, the average half-life, the levels of protein binding, and the effective concentrations needed for treatment. However, every patient is unique. Changes in the gut (if the drug is taken orally), genetic variations in the liver's metabolizing enzymes, and the functional status of organs (like the kidneys and liver) all affect how a drug will be handled by an individual. Given these variables, it becomes important to take a measurement of the drug in serum so that we can be sure the dose given is leading to an appropriate concentration in the body.
TDM helps to ensure that a dosing regimen is appropriate for a given patient.