It's basically what Danzos said, but -30 isn't just for realism, a lot of replay makers these days use -30 to mix in realism with destruction. It also gives you better control over what you do, instead of hitting uke, than flailing off somewhere.
And some people do say, when you use -30, you can dismember more easily than you can in -9.82, since uke just gets pushed around in the lower gravity, -30 makes him stay still after a hit. (if you catch my drift)