When working on an open source project, you face questions of how to make your work consumable. I've been watching the process in my work on Eclipse OMR, and I've come up with my personal mental model:

Commits delimit atomic changes to the source code. Each commit should build, and contains one state transition for the code. Each commit message provides an opportunity to explain your reasoning for making the change.
Pull Requests group together a set of related commits; For example, a feature, or a group of cleanup commits. These commits can be tested and discussed as a whole, and therefore merged as a whole.

Of course, this model is heavily driven by our Pull Request workflow at work.