If it doesn't "prevent" people from checking in code (in the strictest sense of the term) that breaks lots of other code, it certainly does some combination of slowing them way down, notifying other people way sooner, or forcing them to be a great deal more skilled in how they check in their breaking code (in the "if you make something idiot-proof, someone will build a better idiot" sense of skilled).

It's not perfect, but it's better than most of the alternatives, and perfect isn't actually on the table anyhow.