Why have a legal/ justice system? Let karma take care of things?

We all know people who are living immoral lives committing crimes but getting away by bending the law towards their side due to wealth and influence. When ever I see or hear something like this I keep telling my self that karma will get them even though the legal system cannot. But then again I keep question as to why we need a legal system at all when we know that there is a natural system or justice called karma?