Assuming you know Magento well, if you implement your changes directly on your Magento system without tests, you will feel quite productive.
when you don't have exposure to tests, it is a learning curve to use them
if you don't use tests though, then you are liable to test manually your changes. And more importantly, you have to tests the other parts of the system are not broken. (who can claim has never been surprised to find a small change to break something at the other end of the system and found out 8 months down the line..).
--> that means manual tests are not reliable and are not scalable
Below are some thoughts on how to get to test (one day) your code (not too worry, I am only slightly further than you are..
however, you will have to start somewhere, right? there are courses out there on unit tests that are outside of Magento and do a great job of making you profficient with the concept
--> meaning point 1 is easy to get out of the way
point 2 is more complex because we all work on legacy Magento systems and it is not easy to change our way of writing. But following best practices or learning through online portal like
mage2.tv can help as these do use good way of writing code
point 3 is quite subtil and only experience will fill this gap, so no need to worry about now.
Correct answer by Herve Tribouilloy on February 21, 2021
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