How does periodicity checking in the Mandelbrot Set work?

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I was going through the Wikipedia page, on how periodicity checking works in the bulbs of the Mandelbrot Set. I do not understand why the code stores of a new x and y value every 20th iteration. Isn't this not useful, when the period of a bulb is more than 20 ?

Here is the link to the wikipedia page:

https://en.m.wikipedia.org/wiki/Plotting_algorithms_for_the_Mandelbrot_set