The context of a presentation
Usually the narrator knows more about the subject than the audience and in his presentation he is more above the subject. And often the narrator will have a limited time for a presentation, so he or she has done everything to keep the story concise and easy to understand for an interested (lay) audience.
By using compelling images (photos, graphs, diagrams, infographics or a conceptual model illustrating a research design), concise bullet points, summaries, conclusions and if time permits discussion points. Often, after the presentation, you will be asked if the presentation is 'online somewhere'. If so, SlideShare is the place where the presentation can be found online.
The presentations you find on SlideShare are usually carefully made, for a work meeting, a conference or a workshop. The added value of most presentations is that it is not about experiencing moments like with microblogging, but about experiencing a list to data delimited structured story in the form of a presentation.
These features ensure that the emphasis is on content sharing, and the content on SlideShare is different and perhaps often 'richer' than on other content sharing sites (Flickr, only images: photo sharing) and YouTube (moving images, sound: video sharing). A presentation tells a story more explicitly, not only through the combination of images and text, but also because the story is presented as a coherent whole, more or less polished, as a completed story.