I finally found a use case for git submodules: We have a git repo which provides a wrapper around compiling and packaging the Linux kernel for RISC-V, and we use a submodule to link to a specific release of Linux. (https://github.com/rwmjones/fedora-riscv-kernel)

However this also reveals the awkwardness of submodules:

* Everyone who clones the git repo either forgets or doesn't know that they have to do 'git submodule init' (and maybe update too? even I'm forgetting ...). So everyone asks us on IRC why it "doesn't work" and has to be told what to do. IMO git clone should also clone the submodules and set things up.

* You cannot add downstream patches to Linux this way. For a while we needed a small bug fix which wasn't in the Linux submodule (it's controlled by another company) so we had an awkward workaround in the build system to copy the whole submodule and apply the patch before building.

* The link is to a non-fast-forward branch of Linux, so the commit hash sometimes becomes invalid, which we don't notice until someone is trying to build it from scratch. You'd think that because both the module and submodule are hosted on github, that github wouldn't garbage collect the old commit hash, but that's apparently not how it works.

I don't know if there's some tool which solves this use case better, but git submodules are what we have.