I've worked with Mercurial repos for 10+ years and only recently we've been migrating some of the smaller repos to Git primarily due to better supported tooling. Mercurial has done a great job in specific workflows especially with phases (which not everyone adopts) -- by default they disallow modifying public commits but allow modifying draft commits. I don't believe Git really has a similar concept, or at least not integrated at the same level. I recently read about `git push --force-if-includes` which seems like it's trying to address similar situations but mostly guessing based on comparing changesets present between two repos.

To your question about what/how Mercurial helps I'm guessing it's related to this, or other workflows enabled by Mercurial's Evolve/Topic features. It sounds like Sapling has adopted Evolve/Topics and making them functional on Git repositories though I'm not sure what it's doing under the hood.