What properties of radioactive isotopes make them useful?

The answer to this question would be: Uses of radioactive isotopes include tracing the steps of chemical reactions and industrial processes, and diagnosing and treating disease.

The radioactive molecule will decay, resulting in a child molecule and release energy. This could be measured and detected so it can be used in tracing reaction or even diagnosing a disease. The ratio of child molecule:parent molecule ratio can determine an object age.