I actually do have a real example of this.
We use -race during our automated testing. The setup for some of our tests involves CPU mining (rapid blake2b hashing). This code definitely doesn't have any races, and it runs waaaaay slower when race detection is enabled. So we could speed up our tests significantly by disabling race detection just for the setup phase.
The question is: For debug builds that explicitly have the "-race" flag passed to the compiler, why would you want to disable race detection for a specific function?