This tool looks like "submodules, but lighter", while repo is "submodules, but heavier". Looks to me like the motivation is for dependencies that are not hard enough to justify a submodule.

Repo seriously sucks to use, but I also can't imagine many tools living up to AOSP-type workloads without being specifically designed for it. My gripe with repo is that it's really hard to pin the entire repo state if you have a bunch of prototype patches across multiple subrepos. I usually end up having to modify the XML directly.