Even as she performed her meticulous fractional crystallizations, the polonium in her raw material was disappearing as a result of its rapid radioactive decay. You can read Marie Curie's description of radioactivity as understood in 1904. In many circumstances isotopes are more effective and safer than surgery or chemicals for attacking cancers and certain other diseases. Over the years, many other uses have been found for radioactivity. Until electrical particle accelerators were invented in the 1930s, scientists used radiation from isotopes to bombard atoms, uncovering many of the secrets of atomic structure.
Isotopes are crucial even for geology and archeology.
As soon as he understood radioactive decay, Pierre Curie realized that it could be used to date materials. Soon the age of the earth was established by uranium decay at several billion years, far more than scientists had supposed. Since the 1950s radioactive carbon has been used to pin down the age of plant and animal remains, for example in ancient burials back to 50,000 years ago.