Permissions? ie different teams with different push/pull rights for different repositories. We have that where I work; for most projects it seems like needless complexity, but some things are more closely controlled; payment handling, ops (where a change could bring down the whole org not just an app), etc. There's also the issue of contractors being brought in, generally they see a restricted subset of the codebase.
A lot of the VCSes that use the monorepo model allow fine grained permissions on various parts of the repo (this necessitates being able to restrictively fetch only part of the repo).
