To me, it seems that the information about the usage of commas in English is for the most part prescriptive. Very few sources actually explain; indeed, I have yet to see any such source.
If you go online and research 'comma uses', you will find lists of 'rules' regarding the usage of commas. To name a few of these rules:
Use commas to separate independent clauses joined by coordinating conjunctions (FANBOYS). E.g. 'Cars will turn here, and coaches will go straight on'
Use commas after introductory phrases or clauses. E.g. "Once you have done your task, you must come to see me immediately".
Use commas before directly addressing someone. E.g. "If I may interrupt, Your Majesty".
etc.
One thing that all of these 'rules' have in common is that the authorities who prescribe them - if authorities they may be called - offer very little evidence to support their claims - indeed, often no evidence at all. If these authorities did offer evidence, then the basis for their rules would most likely be found in the way that the English language is actually written - represented on paper or by some other medium - by its speakers. Yet there is no such evidence in written English. Consider the below sentences, for instance.
Hi John, how are you doing?
If he does it then he's a fool.
She must either go back to bed or get up.
Each of sentences violates one of the so-called rules regarding the usage of commas in English: the first sentence violates the rule that one must use a comma before a word in the vocative case; the second, that one must use a comma after an introductory clause; the third, that one must use a comma to separate independent clauses. Yet only the most tyrannical of linguistic prescriptivists would regard the above sentences as ungrammatical; most people (even many established writers) would see nothing wrong with these sentences.
It may therefore be seen that, in many cases, whether or not a comma is included is a matter of personal preference. There are exceptions, of course, amongst which is the case in which the absence of a comma leads to ambiguity. However, in my experience, many native English speakers neglect this, too, as they do with so many of the other rules. For instance, this sentence
He didn't play golf because he was ill.
is ambiguous, and yet many English speakers would write the sentence without the comma but with the intention of conveying the meaning of the version containing the comma:
He didn't play golf, because he was ill.
That commas are so often optional begs the question of why these rules still exist. Do commas have any syntactic or semantic function in English, or are they optional punctuation marks that prosodically set a constituent apart from the main clause? Does their prosodic 'separation' (for want of a better word) serve a linguistic purpose? To me, this sentence:
If he does it, then he's a fool
is exactly the same as this sentence:
If he does it then he's a fool
in all aspects but this one: after the if clause in the first sentence, there is a slight pause, a pause which serves no linguistic purpose.
If commas do indeed have a syntactic function, then how can one account for their decline? And if commas don't have any syntactic function in modern English, then did they use to? Did commas have syntactic functions in older versions of English? Were commas once obligatory?
TL;DR: In English, do commas play any role in marking the syntax or semantics of a sentence, and if not, did they have syntactic/semantic/other linguistic functions in older versions of English.
As a supplementary extension of this question, I would be interested to know if the comma (or punctuation in general) is facing a similar decline in other languages. If so, are there any explanations for this phenomenon? (I'm not sure if asking 'supplementary questions' is against the policy of this StackExchange. It probably is, and I won't be surprised if I am chided for asking a supplementary question. Nevertheless, I feel that it is suitable to ask it here.)
Thank you.
