My understanding is the following: every time you save a jpeg in a non-parametric editor, there is additional degradation and it is cumulative. So, if you would just open and save it, say, 100 times, without even editing anything, you would degrade it 100 times cumulatively.
With parametric processing, no matter how many times you edit and save the file, you are going to degrade it only once, thus minimally.