Here's some quotes from Mark Shuttleworth's blog post about what Canonical/Ubuntu does for the upstream communities:
Nevertheless, the Ubuntu Project does bring something unique, special and important to free software: a total commitment to everyday users and use cases, the idea that free software should be “for everyone” both economically and in ease of use, and a willingness to chase down the problems that stand between here and there. I feel that commitment is a gift back to the people who built every one of those packages. If we can bring free software to ten times the audience, we have amplified the value of your generosity by a factor of ten, we have made every hour spent fixing an issue or making something amazing, ten times as valuable. I’m very proud to be spending the time and energy on Ubuntu that I do. Yes, I could do many other things, but I can’t think of another course which would have the same impact on the world.
What do we do for free software? And what do I do myself?
For a start, we deliver it. We reduce the friction and inertia that prevent people trying free software and deciding for themselves if they like it enough to immerse themselves in it. Hundreds of today’s free software developers, translators, designers, advocates got the opportunity to be part of our movement because it was easy for them to dip their toe in the water. And that’s not easy work.
We fix it, too. Consider the https://wiki.ubuntu.com/PaperCut Papercuts project, born of the recognition that all the incredible technology and effort that goes into making something as complex as the Linux kernel is somehow diminished if the average user gets an incomprehensible result when they do something that should Just Work. Hundreds of Papercuts have been fixed, across many different applications, benefiting not just Ubuntu but also every other distribution that ships those applications...At a more personal level, a key thing I put energy into is leadership, governance and community structure.