Surely, by the time something gets committed to master, a developer has already run all the unit tests before and fixed any errors that might've occurred with their new code.
Or not. Perhaps the developer doesn't have the discipline to do that. Perhaps they forgot.
But the real point is to run the tests on a machine that is not the developer machine. One that is configured differently.
This helps catch out issues where tests and/or code depend on something specific to a developer box (configuration, data, timezone, locale, whatever).