There are three main reasons people use PowerPoint in their presentations and all three are good reasons not to!
The first reason most people cite is that it helps them remember what it is that they want to say. They can look at the slides and use them as a prompt for their presentation. The most obvious manifestation of this are the presenters who helpfully read the slides to their audience and then wonder why people are losing the will to live. Problem is I can read silently quicker than you can read out loud and the repitition isn't helping it stick. The less obvious manifestation is that the PowerPoint system becomes the entire template for the presentation. We are stuck with having to listen to someone speaking in a sequence of Titles and Bullet Points and storytelling (which is fundamental to effective human communication) goes out the window. If any of this is the reason you use PowerPoint then either start talking about things you actually know about or prepare better. As an audience we are disengaged and convinced you have a surface understanding of your subject.
The second reason people use PowerPoint is because they think it makes their presentation more interesting and engaging. Nothing could be further from the truth. When the visual source of information is different to the auditory source of information the human brain can't effectively process the two at once. The result is that one will take precedence over the other - usually the visual over the auditory. In other words, I am looking at your slides and I am not listening to you so you might as well not be there. Same thing applies beyond PowerPoint - never go through a proposal with a client. Either give it to them well in advance and then discuss or discuss it first and then leave it with them when you leave. When it comes to PowerPoint it is a case of Show OR Tell not Show AND Tell.
The third reason is perhaps the most common and by far the worst. Most people use PowerPoint because they think they have to. Everyone else does it they think so I guess I better do it to. It's just not a good reason to do anything. The bottom line is that PowerPoint is a form of electronic wallpaper and like wallpaper its primary purpose is to look pretty. Beyond that it really serves no practical purpose.