I think the thing is that there are many places where a comma is optional or a matter of style.
Apart from in lists, the most normal use for me is where it splits a sentence to show a supplementary or independent part of that sentence, but that's just off the top of my head. For example, in my previous sentence if you deleted the phrase before the first comma or deleted the phrase after the second comma then it would still make sense as a sentence.
Oh yes, i did google. As always, it left me more confused so many sites saying different things. You know when you overgoogle until you dont know anything at all? Even things you knew before you started.
To separate items in a list: apples, bananas and pears
To clarify a sentence that would otherwise mean something else: 'i love cooking my family and my dog' means something very different to 'I love cooking, my family and my dog'!
To separate a subordinate clause in a sentence: the girl, who wore a red coat, loved Mumsnetting on a Friday. 'Who wore a red coat' is the subordinate clause - you don't need it for the sentence to make sense.