I often say that slides are a safety net for the presenter in the early days of a presentation. After a number of runs, the presenter becomes confident enough to deliver the story pretty much without slides. Putting up the next slide is merely a mental placeholder that triggers the next point in the story.
As a presenter, you might fail to notice that after a couple of months your story can change/drift, and the actual slide that you put on the projector no longer back it up completely.
It is good to do an objective 10,000 km check up now and then, maybe with the help of a person who is not that immersed in the story as you are.