This is the very basic requirement to start with the testing phase. We should have the test cases for testing any system. test cases is written based on the scenarios you have to test the system. Test cases gives us the assurance that flows are working fine or not based on the status of the test case. If the status of some test cases are failed meaning some flows are not working fine and there is bug in the system that needs to be fixed. If the status of the test cases are passed meaning flow is working as expected and system is behaving as per the requirement.