Well, it means, that the viewer can read the sign, which is quite practical. For example, in Czech dubbed TV series (like The Simpsons), a narrator has to read every sign that has any meaning to the plot, which would be quite weird, if you weren't used to it, and then even surprised that this is not the case in the original version. (As a bonus, it makes the video even more redundant, so you can take the whole episode, rip the audio track, and just listen to it :-))