Tests are necessary to be able to rely on that a certain functionality behaves in a certain way later in production. In all kinds of manufacturing businesses, tests are a natural part of the process. A car has countless parts that need to be tested independently as well as interdependently. Nobody wants to drive a car which has its first test run on a real street with the actual customer.

Tests simulate production behavior and verify components in a safe environment. Manufactured parts that break during test runs are something positive; they have just pointed out potential errors and nothing more than time and materials is lost. Parts that break in production can cause more harm.

The same is true for software tests. Test failures are something positive, at worst they used up some time and effort, at best they prevent potential bugs from going to production.

As seen previously, tests need to run with the least required human interaction possible. Humans are good at thinking about reasonable test cases and crafting creative test scenarios. Computers, however, are better at executing them. Verifying complex tests is something computers also do well, after they have been given clear verification instructions. With software growing more complex over time, the effort of manually verifying behavior gets bigger and bigger and more prone to error over time. Computers perform better and more reliably at repetitive tasks.

Reliable automated software tests are a prerequisite of moving fast. Automated tests can be executed many times, verifying the whole application. Builds run many times a day, execute all tests every time - even if only minor changes were introduced - and enable verified versions to go to production. This would not be feasible with tests executed by humans.

Automated tests increase the reliability of and confidence in the Continuous Delivery process. For Continuous Deployment, that is, going directly to production, sufficient, automated test scenarios are absolutely required. When all commits are potential candidates for production deployment, all software behavior must be adequately verified upfront. Without this automated verification, Continuous Deployment wouldn't be possible.