Every code path covered by a passing test is a path you can rule out during debugging. Code paths grow exponentially with code size, making root-cause analysis combinatorial. A good test suite eliminates most of the haystack, leaving a manageable set of plausible causes where you can form a directed hypothesis rather than speculating blindly.
Unit tests have value even for code you’re confident is correct today, because they pre-invest in debugging speed for the inevitable production issue tomorrow. Refactoring safety and documentation are the more commonly cited benefits, but pruning the debugging search space is the one you’ll appreciate most at 3 AM.