The tests provide a useful confirmation step. You make a change and the tests go "Hey did you know that this affected the output in these 3 places?" and most of

The tests provide a useful confirmation step. You make a change and the tests go "Hey did you know that this affected the output in these 3 places?" and most of the time you did know that and update the tests, but often you see a change you didn't actually expect and can now look deeper in to it. I have also found immense value in tests that just check that literally nothing has changed. Like one that checked that the columns of a db table have not changed and when they have, the test reminds you that you need to decide if the new column is blacklisted from a certain feature and then add the column name to the test so it passes again.