80% of bugs fits into 3 categories

  1. The behaviour is not as expected.

Reason: developers shipped without tests
or
They tested what was not specified clearly.

  1. Bug or feature?

Reason: developers implemented some extra behaviour where there were no specification.

  1. The usage of some dependencies is buggy.

Reason: developers shipped without integration tests.

Test should be part of the coding activity, otherwise you will pass most of your time/most of your money on bugs
AND
When the application grows, fixing a bug without automated tests will surely create at least another bug most of the time.

#Automated tests matters