A mic that’s in good condition and working properly can help an instructor be heard more easily. This is important in classes where most of the people speak the same language and there are auditory learners in the class.
Zumba, for example, is a visual format that doesn’t typically use a microphone (unless a gym requires it). I don’t use a mic for Zumba because I have people who speak 6 different languages in my Monday morning class. In that case, much more effective to teach with my hands and head.
The other thing to consider with a mic is that although it makes one’s voice easier to hear, the intelligibility / clarity / skill of the cues is equally important.