I don't understand the purpose of package managers in general during the development where you have the total ownership of your code, 3rd party dependencies and

I don't understand the purpose of package managers in general during the development where you have the total ownership of your code, 3rd party dependencies and whole build process. I had a privilege to use conan on semi-large codebase - what a clusterfuck that is. I can't imagine a single scenarios why I would be using it.