You get lots of training, plus access to lots of "stuff" that has incredible value on the street. (What would be the street value of a crate of full-auto M-4's?)
This was covered on the GANGLAND series on the history channel. They showed a gangbanger who was ex-marine in a shootout and showed how his training made him much deadlier. They went on to verify that some gangs actually send their members into the military to get trained and see what they can send home that would better the gang. Kind of like using the military as a school for higher education for violence...