They are very useful when you've got hundreds of engineers working on distinct projects that are loosely connected. Imagine you have a team working on core libraries, a few product platform teams, and finally a team working on a customer facing feature. The feature team commits to master and a build kicks off (build 1), that build fails for an unrelated issue (say the build node dies) and the build kicks off again (build 2). With multiple layers of libraries supporting the feature team, its entirely possible that a dependency could have changed and the end result of build 2 would be different than build 1.

When each commit shares a common timeline it is really easy to rebuild build 1 with the exact same dependencies.