The Closure Templates are for HTML and UI elements, and should be not only easy for developers to use but also to localize. This is pretty important for projects that hope to serve an international audience.
All of the Closure bits are licensed under the Apache License 2.0. Having external access to Google's tools -- obviously a set of high-quality tools that are production ready -- is good in and of itself. The fact that they're now open source means that the rest of the community can join in the heavy lifting and improve them, even for Google's competition.