They are small and convenient. They are often used by those with no hearing aids. A variety of coupling devices are available to bring the sound into the ear. These include headsets, ear buds, neckloops and silhouettes. Some listening attachments are more appropriate or effective than others.
Microphones can make a big difference in quality of sound.
They are often the most expensive part of a system. Directional mics might be good for focusing on picking up the speech from a talker. Am omni directional mic picks up sound from various angles and might be helpful during a conference. Consideration needs to be given to number and placement. Microphones ideally should not be stretched for more than 3 people to a mic.