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It's recommended that 70% of the application tests must be done in the unit level
Unit tests helps on providing quick feedback to the team when some change brokes the system
And with unit tests we can obtain a very important metric for software quality, the code coverage
In one way or another, complex systems integrates with each other
And in the case a service stops to respond, we expect that the application do not stops to work
The integration tests are there exacly to help you on the application design, to not broke the system in this kind of situation
With a unit and integration test base, came the UI tests
These help you to provide feedback about the application usage the more closely the user's reality
Besides that, UI tests, also known as end-to-end tests, may be ran together with visual regression tests
Testing all the application layers helps on application regression while it evolves to attend specific needs.
So, after the developement of a new feature, a bug fix, or an application refactoring, you're safe about the already implemented functionalities are still been tested.
Then, your team can continuously improve.