Oftentimes there's some optimization that it'd be really nice if you could perform, but it's possible to write code that the optimization would result in a miscompile of without analysis that's too hard to do without killing compile times. (And often, popular large frameworks do all these terrible things hundreds of times per second!)

By compiling to a VM like the JVM, you expose the opportunity for speculative optimizations to run and be falsified at runtime without affecting correctness. This isn't a silver bullet, but can certainly be useful for an optimizing compiler developer.