Some very famous individuals often hired other people to speak in their place. King George VI of Britain, for example, stuttered quite a bit, and was terrified of public speaking. Hence, for a very long time, he hired an excellent radio voice to deliver his speeches in his place. If you are in a position of power and/or authority, and you fear public speaking, you might hire a public speaker to deliver your speeches in your place. Here are some other good reasons why you might want to hire a public speaker to speak for you.
You Cannot Speak
If you have a physical impairment that disallows you from speaking at all, you are in a tight position. Since most people would expect you to speak, you would have to hire a "voice" to speak for you. Problems with your vocal chords or the musculature of your face and jaws probably makes public speaking physically as well as emotionally uncomfortable for you, but at least you live in a day and age where people are more understanding of disabilities and will not mind the fact that someone accompanies you and acts as your mouthpiece.
You Are an Agoraphobe
It is not unheard of for wealthy or famous people to be terrified of people in general. If you are invited to multiple conventions, conferences or events as a public speaker yourself, but cannot even think of stepping foot outside your work space and your home, a "voice for hire" is ideal. You may also be allowed to speak via internet video modes, if the people who invite you to speak are well aware of your disabling mental health condition, but usually it is perceived as more professional and thoughtful to send someone to speak in your place.
You Have Trouble Pronouncing or Enunciating Words Clearly
If you are invited to speak to a group on foreign soil, but you have difficulty pronouncing words in their language or enunciating clearly, hiring a public speaker in your place is a good idea. He or she can read off your speech cards exactly what you have written, and maybe even add some flourishes to make the crowd laugh. You could also hire a foreign language public speaker who can translate for you, which allows you to deliver your entire speech in your native tongue while your hired speaker translates it word for word to the audience.