[dredmorbius]

All valid motivations. Possible others:

4 - Keeping in practice / learning skills requires constant exercise. Programming problems represent one form of this. Where collaboration itself is a trainable / maintainable skill, public group effort is directly relevant.

5 - Where skill asssessment is difficult, public work product and process function as a shingle and credential.

6 - Collaboration builds and strengthens professional ties and bonds.