Every change risks breaking something you are not looking at. An automated suite turns "I think it still works" into a result you rerun after every edit, composer update and PHP upgrade, and unlike a comment, a test fails the day it stops being true. Aim for a pyramid: hundreds of unit tests, each exercising one class with its collaborators replaced, in microseconds; fewer integration tests proving your SQL against MySQL 524 (Subsection 4.20.12); and a handful of slow end-to-end tests over HTTP. Write a test that reproduces every bug before you fix it, and the bug never returns.