This is a bar of sound. Look at all the colors!! It captures your eye because it's so colorful, not to mention capturing your ear! What if it were all one color, do you think it would still be interesting? I doubt it... in fact, we've all been to those meetings and attended those monotonous talks where the color (so to speak) is all grey, and stays that way throughout. Do you actually retain any of the information from those meetings or talks? Wouldn't it be a win-win if the speaker added color to their speech? They would feel great that they had an interested and captive audience, and the audience would be happy to be there, listening and retaining!
Vocal dynamics and tonality help your listener connect with you. They also help your listener comprehend and remember what you're saying, when used properly. They are a tool for you to land important words or phrases on your audiences ears, helping them pick up on those important ideas, so they don't have to sift them out on their own.
When you speak, use all of the tones of your voice; the highs, the lows, the really lows and really highs and everything in-between. Why would you use the lows of your voice? How about when you need to portray a calming effect, or a stern warning effect, or something definitive perhaps? What about the highs of your voice? How about when you want to portray something victorious, or give the effect of something going terribly wrong and it's chaotic, only to then go low, calming the chaos down?
Your vocal tonality will support the feeling you want to give your audience. Start listening to speakers you connect with and pick up on how dynamic they are, and listen to how they use their vocal dynamics!