Source of nitrogen? Well now, that's interesting....
I read, maybe at the beginning of last year, that it was good for tomatoes since it was acidic and tomatoes like acidic soil.... I told that to my wife recently and she immediately looked up a source that pretty much directly contradicted this (tomatoes *don't* like acidity, it said).
Perhaps the nitrogen helps to make it a happy environment for yeast - aside from sugar, yeast likes various nutrients; nitrogen is one of those.