There are some cases where you have to make a breaking change, and in those cases, it is helpful to be able to assess the fallout without a ton of manual effort. E.g., sometimes a security hole can only be remedied by restricting certain inputs (like with Heartbleed), and that's technically a breaking change.

Monorepos let you make that assessment within a certain scope, as does Amazon's internal build system. It's not a feature you want exercised regularly, but it sure is helpful when you need it.