Before fixing yesterday's bug, because I'd written automated tests, I ran them to ensure they were all passing.
Then, I was able to focus solely on adding the new use case - starting with a failing test to replicate the issue and then getting it to pass.
Because it was already tested, I didn't need to worry about breaking any other functionality and introducing regressions.
When the new test was passing, I could run the whole test suite and ensure they still passed and things continued to work.
Without the tests, I'd either need to check everything else manually (which takes time) or worry that something could potentially be broken.
Having tests meant I could be confident that the new and existing functionality worked.
