The biggest "function" is that the nasty queries don't need to hit your DB server.
As far as cost, I'd say the cost is so minimal, it's nothing more than an arbitrary barrier to prevent people who don't actually need it from trying to set it up (the ones that don't actually need it are going to be the ones that are going to need the most help setting up the backend requirements for it) and making it *not* a support nightmare for Kier and Mike.
As long as you have the JDK set up, installation is disgustingly easy.