When an Ubuntu version is released, it features get frozen. The updates provided for it is mostly security updates and patches. But major changes of software will not be released (included in the repository) for this release.
That is why the default enabled Ubuntu repositories include *security-updates*, *updates* and *proposed* repositories. The first two repositories include bug-fixes and security updates and *proposed* is used for testing the newer updates.
Mainly the backports repository's function as said in the [Wiki][1] is --