The usual reason I’ve heard quoted for using Open source software is the ‘many eyes’ one i.e. security is improved by having more people looking at the source.
While this is true it’s not the only benefit.
I’m working on an Umbraco based project and initially couldn’t easily find the correct way to call part of the api.
So I cloned the Umbraco repo, checked out the particular commit, which was the tag for a particular version, and had my solution and the Umbraco one open. I could then stick a breakpoint in my code and follow it into the Umbraco source and check what happened.
This saved loads of time reading documentation or checking Stackoverflow and the developers comments are often informative and on occasions funny.