"The organizational setup varies between Open Source Software projects, but in some form, people always play user, contributor, and committer roles. Users use the software, contributors contribute in some form, and committers decide what contributions to accept into the project." (http://www.riehle.org/computer-science/research/2007/computer-2007-article.html)
"Committers determine where the open source project is headed, strategically and on a day-to-day basis. They can typically resolve technical problems faster than noncommitters, and have high visibility to the user community. Most projects are set up so there's only a small inner circle of committers, a larger set of contributors, and an even larger user community.
For an employer, the value of employing a committer is manifold. Through the committer, an employer
- gets problems with the open source software fixed faster and better,
- can better align company strategy with the open source project and vice versa,
- appears as a more attractive employer than competitors who don't employ a committer, and
- has higher visibility with the user community and can reach out more effectively.
See the related article on Maintainers