It makes animation and cutsie forms out of a moderately small program. Also, every language is operating system independent. This isn't something unique to Java. C is OS independent. You can write a C program and recompile it for multiple OSes.
People can write portable code.
It's been written for years. But again, this doesn't help you write the program. Instead of working to the lowest common denominator, most large programs wind up using unique features of each platform. Our applications on the Mac try to take advantage of neat features on the Mac. And on Windows we do custom work.