As I’ve mentioned before, a lot of comma usage is a matter of individual style and taste. Writers and their editors (and later in the process, their copyeditors) have had many a wrangle about exactly which commas in a particular work need to stay, and which to go. As far as who wins — well, it’s an ongoing struggle, and over time the honors are about even.
There are, however, a couple of places where the commas are not just a good idea, they’re mandatory. (One caveat: I’m talking here about standard American English usage, because that is my native idiom.) For example:
Standard dialogue punctuation. If a sentence followed by an attribution to a speaker would have ended with a period if it wasn’t in dialogue, then in dialogue it has a comma followed by the closed quote followed by the attribution.
“This is how you do it,” the writer said.
“Do what?” the reader asked.
“Punctuate dialogue, dammit!” said the writer.
Nouns and phrases in apposition to other nouns. Here you have sentences with descriptive words or phrases stuffed into them, like this:
Bob’s uncle, a traveling salesman from Indiana, retired to Jacksonville, Florida, to raise alligators.
Note that the commas in this case always travel in pairs — unless the appositive falls at the end of the sentence, in which case you have sentences like these:
Bob introduced me to his uncle, a retired traveling salesman. His uncle raised alligators on a farm near Jacksonville, Florida.
It’s not a difficult thing to learn, it’s just a pain to remember if you haven’t already gotten the patterns burned into your brain. As with most things, practice helps.
Dr. Doyle's Blog 