I had a system in C, with a goal of extremely low latency. This involved a small collection of threads, each serving a different role. Project goals would periodically change, and I would find myself needing to move some functionality from one thread to another. With some creativity, I was able to enlist the help of the type checker in assuring that a particular function ran only on a particular thread. This sped me up tremendously, as my tooling would point me to inconsistencies rather than having to reason them out myself.