It's important because by preparing you can be sure that you have included all speaking points that you planned to touch upon and so that you don't forget something that you wanted to bring up. It's also important because if you didn't prepare you might forget parts of your speech and there's nothing worse in a presentation than having someone shuffling through notecards or stumbling on their words in front of whoever is judging/grading them.
Whenever I do presentations for my classes I always have to run through them a few times during the days leading up to the actual presentation or else I get really nervous and leave things out and have to backtrack. It helps me so that I know no matter how much stage freight I get, I can remember what I have to say and then focus more on looking and behaving profesionally.
