The first thing is hermeticity and and what it implies: caching. That if targets are a strict function of inputs, and inputs can be hashed, then you can reliably cache them - including test results!

The second thing is distributed caching. Done right, not only are your test results cached, but CI's test results can be cached too.

The third thing is distributed builds. This only starts to matter in big projects, but compilation is inherently a spiky load and if you can share a big pool of compute between a big pool of engineers, you get higher hardware utilization and lower latency to build artifacts.

The fourth thing, something that isn't really feasible outside big tech, is you could be bazel all the way down in a big monorepo. One of the niftiest things at Google is to be able to put a printf inside a database server and run your client test, and blaze knows that it needs to rebuild the database server and it will do it automatically, so that you can get extra insight at almost any level in the stack.