When preparing a speech to a group of people there are, only we should do a few things in preparation at the very beginning. One the most important one of you is set clear goal for our presentation. We should know what we want. What change do we in our audience minds and hearts to make. 'Mental what place' we want to go. Simplify this task for you, I will describe briefly most common target categories. This may help you, find your own presentation purpose.
Conviction - is to make simple something. Believe in something, to do something or to stop, to do something. It is particularly important to prepare well in the art of seduction. The more you know and understand current beliefs of your audience, the easier it is to seduce.
Sales - this is a part of persuasion (also most so I thing it deserves separate description). Like in the above point is people you are going to influence the key factor are proper research about anyone. We should know what needs and problems does have our audience. Thanks to this, we will talk in able to sell our solution, without much. This is because everything we'll tell you will make sense. What you want will be targeted.
Education - sometimes we need to transmit knowledge we have in our mind in their heads. What is important here is to make this process nice and easy. We need to attract the message. This is especially true when our presentation is abort technical staff, and there are non-technical people in the audience. This attractiveness is influenced by the language we use, our body language, the rhetorical devices to include in the speech and the sound of our voice.
Motivation - This is part of the daily work for many managers, and of course motivational speakers. What is the decisive factor here to tell an interesting story that arouses emotions and brings some news with you. Best scenario be from my own experience.
These four objectives are only most common categories and own personal goal may be somewhere between two of you, can be mixed in only a few of you or maybe something else. I hope that helped you a little with the Mission of your personal presentation purpose.