Applitude

To be agile and stay agile, you need to change things with confidence without risking change that could result in an unpredictable behavior or in a bug. To do this, verify that previously built features are not affected by newly introduced code. If you don’t create automated tests causing you to do a lot of manual work, you incur unnecessary risks when introducing new features.  As a result, your system will become very fragile.

Another argument in favor of several automated tests is that unit testing and test driven development represents the cost of handling bugs. The cost of fixing a bug that has already reached the production environment can be very expensive. Ideally, a bug should never appear.

Practices like unit testing, automated tests, continuous integration, refactoring, TDD, iterations and incremental development and simple design where separation of concerns are applied is great insurance to be able to be agile without becoming fragile. As soon as you start tamper with these practices your risks increase and ability to flexibility decrease.