A comfortable speaker relaxes the audience and makes them more receptive. This comfort flows from your confidence in your message and in your ability to deliver it, both of which can be learned.
Every speaker seeks to sway their audience to action in some way. Effectiveness at reaching that goal requires a combination of appropriate and concise content, good organization, and smooth delivery.
Persuasion means creating action in others, whether it is making a sale, attracting investors, or gaining supporters. There is no more powerful tool to persuade others than a convincing verbal argument.