From a recent book by Harvard Business Press:
The conclusion [in a presentation] should not summarize your arguments; rather, it should appeal to the audience for its understanding, its action, and its approval — whatever it is you want the audience to do or think.
So don’t fall into the trap of telling your audience what you’ve already said. Summing it up is a surefire way to kill any enthusiasm your presentation may have generated. So forget about a summary; instead, tell your audience what it should think or do.