Repetition is important because it can help you practice important parts of your speech. For example, if you are having trouble articulating a word correctly, you can practice it over and over again until you nail it. In public speaking, you must sometimes menmorize speeches, so repetition of key words and phrases could help with that. Furthermore, repetition of data and cards, or counterarguments with a little bit of data on them, can help you know what to say at the right time.
That is why repetition is so important in public speeches.