Much has already been said about the importance of automated tests: they provide a great safety net when modifying system components, alerting to issues much earlier in the development lifecycle. As a result, bugs are prevented from ever reaching production environments.
When we're working with legacy code that has very low automated test coverage (or no test coverage at all), building automated tests can be difficult and frustrating. The initial effort of setting up automated tests is frequently more than the team can afford at the time, and we end up deferring it indefinitely.