The key function of version control systems in collaborative software projects is to manage changes to source code over time. These systems, such as Git and Subversion, track revisions, allowing multiple developers to work on the same codebase simultaneously without conflicts. Version control enables teams to maintain a history of changes, revert to previous versions, and collaborate effectively through branching and merging. This ensures that all contributions are integrated cohesively and helps resolve issues arising from simultaneous edits. Ultimately, version control fosters improved collaboration, productivity, and accountability in software development.