The instruction . is particularly useful when an interesting structure has been found on the general screen, but it is evolving too fast or it is hard to pick out fine details clearly at the resolution of the screen. By returning to the main menu (using any keystroke followed by y) the evolution can be followed one generation at a time, and then recorded by a screen dump or noted by hand on a scrap of paper.
Similarly it is possible to transfer the results of the de Bruijn diagram or any other interesting string to the line menu, then return to the main menu to follow their evolution closely with the dot command.
The instruction which repeats the initial segment of the line can be used to increase the variance in some of the probabilistic studies, and to force the automaton to enter a cycle sooner than otherwise. Nevertheless, for most rules a ring of 20 cells is still much too long to reach a cycle within a thousand generations.