Every drug has a sub-clinical concentration (a concentration at which effective therapy won't be achieved) and a toxic concentration (a concentration at which the drug will be harmful to the patient.)
For some drugs, the range between the minimum effective concentration and the toxic concentration is large. These drugs are thus relatively safe. Other drugs have a very narrow therapeutic window and need closer monitoring. This is the role of TDM.
Medications with narrow therapeutic windows, like the anticonvulsant carbamazepine (Tegretol), should be closely monitored since elevated doses can cause serious conditions such as agranulocytosis.