It is essential for all harmonica players to know how to use a microphone. Even if you mainly in small rooms with only a guitar accompaniment, it is likely that you will be required to use a microphone at some stage.
If you play with a band, you will use a microphone every time you perform. It is a good idea to have your own microphone that you are comfortable with, even if the venue you are performing at provides them along with the PA system.