You want to communicate your message as effectively as possible. An inability to hear easily, without strain or distortion, is an unnecessary hurdle.
In addition, your voice is likely not as loud as you think it is. This is a reason many people choose not to use a microphone. A loud voice, though, seems louder to the speaker than it does to the audience.
Even if your voice is loud and you know how to project well, the change in sound makes the presentation disjointed.