Reasons to contribute

Interest

The first and simplest reason is that a developer is interested in some open source software. If you are curious about how a specific software project is implemented, you could look to the public repository and review the code.

Build your resume

The second use case of becoming a collaborator for open source software is resume building. Many software developers have a hard time starting their career and adding valuable content to their resume. Besides internships, co-ops, and other positions, contributing to open source projects is an easy and free way to get recognition for your work. Employers can view a collaborator's github profile and see the specific contributions a collaborator has made. Collaborators may even contribute to projects to specifically highlight their knowledge base.

Integrating Open Source Projects

Another use case is integration of open source projects. Integration means taking an open source project and integrating it into another project to improve it. You can tweak and even improve the open source project to fit your own needs, and these improvements can then be integrated into the main repository. Many developers do not want to work from scratch and include open source libraries in their projects. This allows developers to write software faster and more reliably.