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Unless you're inserting as the first line ...

The benefit (compared to more classic formatting) only occurs if you're inserting at the end of the argument list.

If adding stuff to the end of a class or of an argument list is your most frequent case, you might have a software design problem.

Is that a benefit?

Yes. When you git blame a line you won't get a commit just adding/removing a comma changing on that line.

Some languages/DSL's allow for a comma at the end of the list too, thereby getting the same result but without the slightly strange comma at the start of the line. Adding additional lines is simple then and doesn't cause the line above to be pulled in to any git action

Maybe git blame should get better.