1. Different virtualization/container engine: Different platforms have different good stuff. I would use LXC/docker for Linux since it's super lightweight, and disposable. Even NaCl should be an option.
2. Standardizing would also help defining how resources should be allocated/re-used on local machines.
3. People who make web apps gets to use a single standard instead of tracking different technologies. (Think about Java Applet vs Flash vs Silverlight) Any site should be able to access it through a port on localhost. It's THE port, using THE protocol. And that's it; it should work on all hosts that have the service installed.
4. It brings a chance to have it accepted by major browsers makes as a core service, which makes it easier for wide adoption.