Confidence. A well designed test suite with a high degree of coverage eliminates, or at least strongly mitigates the fear of change. And when you aren't afraid to change your code, you will clean it. And if you clean it, it won't rot. And if it doesn't rot, then the software team can go fast.
Tests are part of deploys
"This kind of aggressive technical debt cleanup and optimization work can only happen as by-product of our engineering ecosystem. Without being able to deploy the main application more than 60 times in a day, and without the tooling to automatically test and benchmark two wildly different implementations in production, this process would have taken months of work and there would be no realistic way to ensure that there were no performance or behavior regressions."