why I need testings in rails?

You should strive to make your tests automated wherever its realistic
to do so. Rails makes it easy to unit test your model and controller
code, and has functional testing to test your apps end to end
functionality too. Not testing your code properly is a form of
professional malpractice IMO.

Untested code is pretty worthless, tests ensures that your code is
behaving like it is specified, it allows your code for easy
refactoring.

That's the big one for me. It's usually pretty obvious when your code
is or isn't working during the initial development. But when you come
back to it a month later to make a change, it's so nice to have "rake
test" waiting to flag everything that just got broken.

-- James