Commas are a very necessary form of punctuation. Missing them out, using them wrongly or over-using them makes text hard to understand; when commas are missed out, ambiguities creep in; when wrongly used, they disrupt a sentence or phrase; when over-used, they not only disrupt the sentence, but result in 'run-on' sentences, i.e. sentences where commas are used instead of full stops, creating one sentence from what should be three or four sentences.

Here is an example of a run-on sentence:
The sentence should be divided up:
Here are some examples of good comma use.

separating items in a list:
N.B. If the list is long and complicated, it's sometimes also helpful to put a comma before the last item. This is known as a serial comma.
joining two related sentences, the second of which begins with and, or, but, yet or while:
marking the beginning and end of a sub-clause - a subclause is a phrase that provides additional information:
Octavian assumed a new name, Augustus, after winning the battle of Actium.
Prometheus, who remains on stage throughout, provides a fixed point.


introducing or within quoted speech:

If you put commas in the wrong places, or miss them out when they are needed, your work will be hard to understand. This will cost you marks. If you are unsure about a comma, read your work out loud. If you need a pause, you need a comma.

A useful tip: the more good writing you read, the better attuned your ear will become. Eventually, comma placement will be instinctive.