When you write tests for a simple function like add, it might seem enough to check just one or two cases. However, functions often behave differently in unusual or extreme situations. These are called edge cases. For example, adding zero, negative numbers, or very large values can sometimes produce unexpected results if the function is not written or tested carefully.

By including tests for negative numbers and zero, you make sure the add function works correctly in all situations—not just the most common ones. Edge case testing helps you catch bugs early, ensures your code is reliable, and builds confidence that your function will not break when used in unexpected ways.