There are millions of illegal immigrants on welfare, in schools and many tens of thousands sitting in prisons. This isn't exactly adding to the well-being of the nation.
1. The legalization of current workers and the demise of the millions of non contributing underground businesses will pay the freight for those social services currently going to non contributors.
2. The screening process will catch former criminals and current ones still at large and deport them. That should also help some.
As to what it does not, people commit crimes, at least they will be 100% subject to our legal system which will have more resources for the reasons stated above.