Open source is done in a collaborative way and but also a very distributed way. While this works great, it is very beneficial for the core developers of these packages to meet once in a while in person. It lets them iterate more quickly on the code and discuss about core issues that would otherwise require long online discussions.
The CDS supported two coding sprints on the MNE and Nilearn (Machine learning for Neuro-Imaging in Python) packages. The Coding Sprints happened jointly given their overlap in applications towards neuroscience. All the coding sprints were massive successes with developers coming for one week in Paris from all around the world.