When you know a subject well, your mind moves quickly between ideas. The connection between slide seven and slide eight feels obvious because you already know the argument that links them. An audience does not have that advantage.
Silent rehearsal lets those invisible bridges remain invisible. You look at the slide, know what you mean, and move on. Speaking aloud forces you to build the bridge with words.
Breath is a diagnostic tool
Long sentences look harmless on a screen. They become revealing when you try to say them in one breath. So do paragraphs that contain three ideas, caveats that swallow the main point, and transitions that require you to say ‘anyway’ before continuing.
The physical act of speaking exposes density. If the sentence is hard to say, there is a reasonable chance it is also hard to follow.
A listener changes the standard
Presenting to a chair, mirror, or empty room can help. Presenting to one attentive person is different. Their presence creates just enough social reality to make you communicate rather than merely recite.
You notice where you start explaining too much. You hear the moment your confidence drops. You discover which slide you keep apologizing for. None of this requires feedback; the presence of another mind changes how you formulate the message.
Rehearse before polishing
The common sequence is backwards: perfect the slides, then rehearse. A better sequence is to rehearse the rough argument early, while it is still cheap to change.
If the logic survives a spoken run-through, design can make it clearer. If the logic does not survive, prettier slides only make a weak argument more expensive to repair.