In my view, there are actually two distinct things going on in that picture of the Flash. One is the multiple images, which y’all have pointed out. I’d highlight the fact that of the multiple images, one is always more clear than the others, and that clear one is the most recent of these time-lapse images. This helps to establish the sequence of the images, which is always helpful, but especially so if the person is moving backwards, or if the moving object does not have an obvious face.
But the other effect is the lines, and they can appear even when only a single image is used. Even without a second image displaying a previous image of the subject, the artist can use these lines to show motion, with the lines showing where the subject had been before.
In that picture of the Flash, BOTH techniques appear.
(When I was around 7 years old, I went for an eye exam, and I was shown the letter “E” and they asked me which way it was pointing. I asked what he meant by that, but he couldn’t answer other than to repeat the question. It was a very stressful event for me. I saw the “E” quite clearly, but I did not know whether the correct answer was “pointing to the right” because the three horizontal lines radiate out towards the right, or whether the correct answer was “to the left” because I also saw it as a picture of a vertical line moving to the left.)