After a while, you’ve seen that meme so many times about how a comma can save your granny from cannibals, you just ignore it, right? Who cares? Granny is tough and can fight off dozens of cannibals with her Cast-Iron Skillet of Doom (DOOM, I tell you!). She doesn’t need some stupid punctuation mark to help her…
Well, I’m not going to tell you about how a comma can save your granny from cannibals.
I am going to show you a few more examples how a comma really changes the meaning of a sentence that contains a direct address.
“I need space people!”
“I need space, people!”
With the comma there, you don’t have to guess whether the speaker wants more room or more astronauts; you know.
My all-time favorite should-have-used-that-comma for this is one I read in a near-future SF novel a few years ago. The dialogue should have been written as, “Let’s go, mate.” Think about that for a moment, and you’ll see why leaving the comma out was a bad choice.
Would you believe it only now occurred to me that I may need to explain what direct address is? (They used to teach this stuff in school, but the internet does leave one with the impression that such is no longer the case…) Direct address is — unsurprisingly — when someone is addressed directly, either by name or by some other I-mean-you indicator. Dude can be a direct address: “Dude, are you hiding the closet with a knife?”
Compare “I don’t know Marty” and “I don’t know, Marty.” In the first, you’re talking about Marty; in the second, you’re talking to Marty. Your readers should never have to guess which you mean, and you can’t rely exclusively on context to make it clear.