I know, it's hard not to have an exact starting point. I'm a very literal person and just go nuts with being told "just use 'some'!"
Does it work? Yes. But, I was remembering when I first started using it that I didn't notice any difference at all - I put a few drops in my mixing water. I was just getting really into watercolors.
Now, when I look at my paintings from then, I can tell exactly which I used it with - well, the ones that I probably used quite a bit with - they're so much shinier.
I like to have a few drops in my mixing water all the time now. When I paint, I like to make a puddle of water in the space I'm working on (say a petal of a flower) and then float paint into it. I find that when I put Gum Arabic in the water, that puddle stays exactly where it's supposed to much easier - it doesn't seep out once it sinks into paper. I also get harder edges with it - sometimes a good thing, sometimes not.