Simplistically, it just replaced those slides which we used to put on an overhead projector. It also does away with having to write on a board. It allows any speaker to focus more on delivering the oral part of the lecture or presentation. It has revolutionized the field of making presentations, making them more interesting and allows the creator to choose an array of ways for driving home a point. These days, it's not enough that what you say is profound or substantive, you have to be interesting as well. This tool allows enough leverage to elicit at least some visual interest.
Of course, there's a danger that comes with a very good tool. We must remember that presentation tools may be a distraction from the real issues. PowerPoint is generally good for what it does, which is put images on a screen. Obviously, it’s much better than the slide and overhead projectors it was designed to replace.
But that’s just the point: slides are props.