This is a poly-as-mono repo which we created for integration testing of our dart and flutter packages.
There are benefits to both polyrepo and monorepo approaches. In this organization, we've opted to go with a hybrid poly-monorepo approach (several smaller monorepos) which suits most of our development needs. One of the main reasons to opt for a monorepo is for integration testing, and therefore we've created a poly-as-mono repo to make integration testing easier. It is also a useful place in which to keep diagrams of end-to-end user journeys, code paths, dependency trees, etc. which typically span across multiple repos.
Read more in the in-depth explanation.