I was thinking about this design, and I think I've found a compelling usecase:

• by design, cmake does not have a way to install ctests. You can install the googletest, but that's not equivalent due to the usage of labels in ctest. Making it a separate CMake project would allow to call the ctest on already installed packages
• if downstream does not package a binding like fortran, this can more easily be added by the user by importing the sub-project only
• in principle it should be possible to redirect FetchContent/find_package to the top-level project. I'm not sure if FIND_PACKAGE_ARGS is needed, or if we can use export() only. Need to test this out. Worst case scenario, I bundle my back-ports
• could make the top-level CMakeLists cleaner. I am on the fence for this one, we'll need to see how it looks in practice vs how much we obfuscate by moving to different folders