[Arainach]

* Tooling improvements benefit everyone. Maybe that's a faster compiler, an improved linter, code search, code review tools, bug database integration, a presubmit check that formats your docs - it doesn't matter, everyone has access to it. Otherwise you get different teams maintaining different things. In 8 years at Microsoft my team went through at least four CI/CD pipelines (OK, not really CD), most of which were different from what most other teams in Windows were doing to say nothing of Office - despite us all writing Win32 C++ stored in Source Depot (Perforce) and later Git.

* Much easier refactors. If everything is an API and you need to maintain five previous versions because teams X, Y, Z are on versions 12, 17, and 21 it is utter hell. With a unified monorepo you can just do the refactor on all callers.

* It builds a culture of sharing code and reuse. If you can search everyone's code and read everyone's code you can not only borrow ideas but easily consume shared helpers. This is much more difficult in polyrepo because of aforementioned versioning hell.

* A single source of truth. Server X is running at CL #123, Server Y at CL #145, but you can quickly understand what that means because it's all one source control and you don't have to compare different commit numbers - higher is newer, end of story.