One the big difference between open source and closed source projects is that open source projects have a natural tendency to create a community around the project. In the world of closed source software, people have well defined roles, like user, QA, developer, project manager, or program manager. In the open source community the difference between roles becomes less relevant: the same person can one day be a user of the product, the next day find a bug, and a day after that fix the bug and submit a patch to the community.
Because roles are relatively unimportant, everyone can talk to everyone. This shortens the distance between people, in particular between those who are using the product, and know very well about the features they need, and those who spend most of their time implementing new features in the product.
Benefits go both ways: if you have a feature to suggest, you know your voice will be heard, that your request be discussed, and is likely to be taken into account. If you are working on the product, you can get ideas about new features from the community and discuss those ideas with the community.
To a great extent, in an open source project, the community is in fact your project manager.