The main reason behind learning about Testing is to check if the views that we have written for, are generating the expected results or not. Suppose you are writing an APIView to get a list of something, then you should write a testcase to check whether that APIView is giving you the result that you are expecting or not.
Suppose a company has manufactured a laptop and then they test it to check if the new laptop design is working as expected or not. In the same way, if you are doing any development work, be it backend development or frontend development, you are supposed to and must write test cases to check if the feature that you have just developed is giving the expected response or not.
I hope, now, the basic idea of testing is clear to you.