Please give me a source that states this so I won't make the mistake in the future.
Hi, Tabac. I'd happy to hazard a guess at your request. As I'm not an expert at discovering a source, here is my rule of thumb:
either or not,
put a comma in between a lengthy sentence,
whenever a natural pause occurs at any place of the sentence in question,
and we need to take a fast and shollow breath, so that we can go on reading it,
without being out of breath.
Well, this sentence doesn't make much sense, yet it indicates that some of the comas, which show a natural pause, are optional.
Hopefully, this makes some sense to my friends.