why I need testings in rails?

Automated Unit Testing is a powerful technique in driving code quality and
improving confidence in the code.

Developer Unit testing takes up your precious time, that could be used
for getting more coding done, or could be used for more involved testing
that really needs a human.

Automated Unit Testing by definition, works for free and in parallel with
your day to day tasks.

Once you have used it, it is really difficult to go back. Without Automated
Unit Tests there are developer activities like refactoring that you simply
cannot do safely. You can afford to take risks during feature development,
because you are more confident that unintended consequences can be
captured.

I would never by choice commence another project that didn't inherently
use automated unit testing.