Singing on Friday nights and Saturday mornings, as well as on holidays, is only part of a cantor's job description. Cantors today are ordained clergy who perform all the religious duties once confined to rabbis. That means that besides the usual congregational business of conducting weddings and funerals, leading services and visiting sick congregants, cantors typically also direct the synagogue choir and teach classes. And, perhaps most important, they prepare bar and bat mitzvah students. All the while, they must find the time to care for their voices with vocal exercises and lessons.
Vocal problems are a common occupational hazard.
Often congregations hire a cantor instead of an assistant rabbi. So cantors are doing two jobs. I can't afford to get sick. A rabbi can still give a sermon with a sore throat. But I may not be able to sing.