Why do we need tests?

To easily make changes to the code.

When you work with someone else's code, there may be a lot of troubles:

1. The code is unfamiliar. Implementation details are not obvious.
2. The code can be toughly coupled.
3. It can be difficult to track where else the code is used.

Functional tests make life much easier. Not these that just check the code coverage. But those tests that test exactly the behaviour, functionality of a program.

Tests are like a safety net on a construction site. When they are exist, you can cut out or add features bolder. The features will get to the production faster. And it will be beneficial for the company in money.